In software engineering, a project fork happens when developers take a copy of source code from one software package and start independent development on it, creating a distinct and separate piece of software. The term often implies not merely a development branch, but also a split in the developer community; as such, it is a form of schism.  Grounds for forking are varying user preferences and stagnated or discontinued development of the original software.
Free and open-source software is that which, by definition, may be forked from the original development team without prior permission, and without violating copyright law. However, licensed forks of proprietary software (e.g. Unix) also happen.
The word "fork" has been used to mean "to divide in branches, go separate ways" as early as the 14th century.  In the software environment, the word evokes the fork system call, which causes a running process to split itself into two (almost) identical copies that (typically) diverge to perform different tasks. 
In the context of software development, "fork" was used in the sense of creating a revision control "branch" by Eric Allman as early as 1980, in the context of SCCS: 
Creating a branch "forks off" a version of the program.
The term was in use on Usenet by 1983 for the process of creating a subgroup to move topics of discussion to. 
"Fork" is not known to have been used in the sense of a community schism during the origins of Lucid Emacs (now XEmacs) (1991) or the BSDs (1993–1994); Russ Nelson used the term "shattering" for this sort of fork in 1993, attributing it to John Gilmore.  However, "fork" was in use in the present sense by 1995 to describe the XEmacs split,  and was an understood usage in the GNU Project by 1996. 
Free and open-source software may be legally forked without prior approval of those currently developing, managing, or distributing the software per both The Free Software Definition and The Open Source Definition: 
The freedom to distribute copies of your modified versions to others (freedom 3). By doing this, you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.
3. Derived Works: The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
In free software, forks often result from a schism over different goals or personality clashes. In a fork, both parties assume nearly identical code bases, but typically only the larger group, or whoever controls the Web site, will retain the full original name and the associated user community. Thus, there is a reputation penalty associated with forking.  The relationship between the different teams can be cordial or very bitter. On the other hand, a friendly fork or a soft fork is a fork that does not intend to compete, but wants to eventually merge with the original.
Eric S. Raymond, in his essay Homesteading the Noosphere ,  stated that "The most important characteristic of a fork is that it spawns competing projects that cannot later exchange code, splitting the potential developer community". He notes in the Jargon File: 
Forking is considered a Bad Thing—not merely because it implies a lot of wasted effort in the future, but because forks tend to be accompanied by a great deal of strife and acrimony between the successor groups over issues of legitimacy, succession, and design direction. There is serious social pressure against forking. As a result, major forks (such as the Gnu-Emacs/XEmacs split, the fissioning of the 386BSD group into three daughter projects, and the short-lived GCC/EGCS split) are rare enough that they are remembered individually in hacker folklore.
David A. Wheeler notes  four possible outcomes of a fork, with examples:
Distributed revision control (DVCS) tools have popularised a less emotive use of the term "fork", blurring the distinction with "branch".  With a DVCS such as Mercurial or Git, the normal way to contribute to a project, is to first create a personal branch of the repository, independent of the main repository, and later seek to have your changes integrated with it. Sites such as GitHub, Bitbucket and Launchpad provide free DVCS hosting expressly supporting independent branches, such that the technical, social and financial barriers to forking a source code repository are massively reduced, and GitHub uses "fork" as its term for this method of contribution to a project.
Forks often restart version numbering from 0.1 or 1.0 even if the original software was at version 3.0, 4.0, or 5.0. An exception is when the forked software is designed to be a drop-in replacement for the original project, e.g. MariaDB for MySQL  or LibreOffice for OpenOffice.org.
The BSD licenses permit forks to become proprietary software, and copyleft proponents say that commercial incentives thus make proprietisation almost inevitable. (Copyleft licenses can, however, be circumvented via dual-licensing with a proprietary grant in the form of a Contributor License Agreement.) Examples include macOS (based on the proprietary NeXTSTEP and the open source FreeBSD), Cedega and CrossOver (proprietary forks of Wine, though CrossOver tracks Wine and contributes considerably), EnterpriseDB (a fork of PostgreSQL, adding Oracle compatibility features  ), Supported PostgreSQL with their proprietary ESM storage system,  and Netezza's  proprietary highly scalable derivative of PostgreSQL. Some of these vendors contribute back changes to the community project, while some keep their changes as their own competitive advantages.
In proprietary software, the copyright is usually held by the employing entity, not by the individual software developers. Proprietary code is thus more commonly forked when the owner needs to develop two or more versions, such as a windowed version and a command line version, or versions for differing operating systems, such as a word processor for IBM PC compatible machines and Macintosh computers. Generally, such internal forks will concentrate on having the same look, feel, data format, and behavior between platforms so that a user familiar with one can also be productive or share documents generated on the other. This is almost always an economic decision to generate a greater market share and thus pay back the associated extra development costs created by the fork.
A notable proprietary fork not of this kind is the many varieties of proprietary Unix—almost all derived from AT&T Unix under license and all called "Unix", but increasingly mutually incompatible.  See UNIX wars.
Berkeley DB (BDB) is an unmaintained embedded database software library for key/value data, historically significant in open source software. Berkeley DB is written in C with API bindings for many other programming languages. BDB stores arbitrary key/data pairs as byte arrays, and supports multiple data items for a single key. Berkeley DB is not a relational database, although it has advanced database features including database transactions, multiversion concurrency control and write-ahead logging. BDB runs on a wide variety of operating systems including most Unix-like and Windows systems, and real-time operating systems.
Free software or libre software is computer software distributed under terms that allow users to run the software for any purpose as well as to study, change, and distribute it and any adapted versions. Free software is a matter of liberty, not price; all users are legally free to do what they want with their copies of a free software regardless of how much is paid to obtain the program. Computer programs are deemed "free" if they give end-users ultimate control over the software and, subsequently, over their devices.
GNU is an extensive collection of free software, which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operating systems popularly known as Linux. Most of GNU is licensed under the GNU Project's own General Public License (GPL).
ksh) is a Unix shell which was developed by David Korn at Bell Labs in the early 1980s and announced at USENIX on July 14, 1983. The initial development was based on Bourne shell source code. Other early contributors were Bell Labs developers Mike Veach and Pat Sullivan, who wrote the Emacs and vi-style line editing modes' code, respectively. KornShell is backward-compatible with the Bourne shell and includes many features of the C shell, inspired by the requests of Bell Labs users.
XFree86 is an implementation of the X Window System. It was originally written for Unix-like operating systems on IBM PC compatibles and was available for many other operating systems and platforms. It is free and open source software under the XFree86 License version 1.1. It was developed by the XFree86 Project, Inc. The lead developer was David Dawes. The last released version was 4.8.0, released December 2008. The last XFree86 CVS commit was made on May 18, 2009; the project was confirmed dormant in December 2011.
XEmacs is a graphical- and console-based text editor which runs on almost any Unix-like operating system as well as Microsoft Windows. XEmacs is a fork, based on a version of GNU Emacs from the late 1980s. Any user can download, use, and modify XEmacs as free software available under the GNU General Public License version 2 or any later version.
Pine is a freeware, text-based email client which was developed at the University of Washington. The first version was written in 1989, and announced to the public in March 1992. Source code was available for only the Unix version under a license written by the University of Washington. Pine is no longer under development, and has been replaced by the Alpine client, which is available under the Apache License.
TeamForge is a proprietary collaborative application lifecycle management forge supporting version control and a software development management system.
The GNU Autotools, also known as the GNU Build System, is a suite of programming tools designed to assist in making source code packages portable to many Unix-like systems.
GForge is a commercial service originally based on the Alexandria software behind SourceForge, a web-based project management and collaboration system which was licensed under the GPL. Open source versions of the GForge code were released from 2002 to 2009, at which point the company behind GForge focused on their proprietary service offering which provides project hosting, version control, code reviews, ticketing, release management, continuous integration and messaging. The FusionForge project emerged in 2009 to pull together open-source development efforts from the variety of software forks which had sprung up.
In the 1950s and 1960s, computer operating software and compilers were delivered as a part of hardware purchases without separate fees. At the time, source code, the human-readable form of software, was generally distributed with the software providing the ability to fix bugs or add new functions. Universities were early adopters of computing technology. Many of the modifications developed by universities were openly shared, in keeping with the academic principles of sharing knowledge, and organizations sprung up to facilitate sharing. As large-scale operating systems matured, fewer organizations allowed modifications to the operating software, and eventually such operating systems were closed to modification. However, utilities and other added-function applications are still shared and new organizations have been formed to promote the sharing of software.
OpenKM is a document management system that provides a web interface for managing nonspecific files. It has a Free/Libre Community Edition, and a proprietary Enterprise Edition. OpenKM includes a content repository, Lucene indexing, and jBPM workflow. The OpenKM system was developed using open technology.
Companies whose business centers on the development of open-source software employ a variety of business models to solve the challenge of how to make money providing software that is by definition licensed free of charge. Each of these business strategies rests on the premise that users of open-source technologies are willing to purchase additional software features under proprietary licenses, or purchase other services or elements of value that complement the open-source software that is core to the business. This additional value can be, but not limited to, enterprise-grade features and up-time guarantees to satisfy business or compliance requirements, performance and efficiency gains by features not yet available in the open source version, legal protection, or professional support/training/consulting that are typical of proprietary software applications.
GNU Emacs is a free software text editor. It was created by GNU Project founder Richard Stallman, based on the Emacs editor developed for Unix operating systems. GNU Emacs has been a central component of the GNU project and a flagship project of the free software movement. Its name has occasionally been shortened to GNUMACS. The tag line for GNU Emacs is "the extensible self-documenting text editor".
Drizzle is a discontinued free software/open-source relational database management system (DBMS) that was forked from the now-defunct 6.0 development branch of the MySQL DBMS.
A free-software license is a notice that grants the recipient of a piece of software extensive rights to modify and redistribute that software. These actions are usually prohibited by copyright law, but the rights-holder of a piece of software can remove these restrictions by accompanying the software with a software license which grants the recipient these rights. Software using such a license is free software as conferred by the copyright holder. Free-software licenses are applied to software in source code and also binary object-code form, as the copyright law recognizes both forms.
The GNU General Public License is a series of widely used free software licenses that guarantee end users the four freedoms to run, study, share, and modify the software. The license was the first copyleft for general use and was originally written by the founder of the Free Software Foundation (FSF), Richard Stallman, for the GNU Project. The license grants the recipients of a computer program the rights of the Free Software Definition. These GPL series are all copyleft licenses, which means that any derivative work must be distributed under the same or equivalent license terms. It is more restrictive than the Lesser General Public License and even further distinct from the more widely used permissive software licenses BSD, MIT, and Apache.
KDE Projects are projects maintained by the KDE community, a group of people developing and advocating free software for everyday use, for example KDE Plasma and KDE Frameworks or applications such as Amarok, Krita or Digikam. There are also non-coding projects like designing the Breeze desktop theme and iconset, which is coordinated by KDE's VisualDesignGroup. Even non-Qt applications like GCompris, which started as a GTK-based application, or web-based projects like WikiToLearn are officially part of KDE.
The History of the Berkeley Software Distribution begins in the 1970s.
Forks are a natural part of the open development model—so much so that GitHub famously plasters a "fork your own copy" button on almost every page.See also Nyman, Linus (2015). Understanding Code Forking in Open Source Software (PhD). Hanken School of Economics. p. 57. hdl:10138/153135.
Where practitioners have previously had rather narrow definitions of a fork, [...] the term now appears to be used much more broadly. Actions that would traditionally have been called a branch, a new distribution, code fragmentation, a pseudo-fork, etc. may all now be called forks by some developers. This appears to be in no insignificant part due to the broad definition and use of the term fork by GitHub.