Open source games are games that have had their source code officially released to the public. This allows users to modify and redistribute the code. One benefit of open source games is that they allow a game's enthusiasts to create source ports.
While some game developers have developed and released their code and entire game content under open source licenses, others have released only the game engine code. For example, John Carmack is known for releasing the source code for most of id Software's older commercial games under a GNU General Public License. However, while their source code is freely available, the actual game content (such as art, sound and intellectual property) remain proprietary.