Free games, also called libre games, consist solely of free software and free media (free as in freedom). This essentially means that the game as a whole can be legally used, studied, modified and shared by anyone, for any purpose, even commercial (sometimes under conditions that don't infringe on these rights).
Free software is software that respects users' freedom and was originally defined by Richard Stallman and his GNU project. Free in this case does not mean gratis (free of cost) – for that we use the term freeware. The term libre is sometimes used as a synonym to free to prevent this confusion. To understand the concept of free software, think "free speech" not "free beer".
The term free software is similar and related to the term open source, but they are not absolutely synonymous. In essence, free software has ethics and user's freedom as a goal, while open source is more lax as it abandons any talk about ethics and focuses on business.
Free media are all kinds of art works (images, videos, data, sound effects and other game content) that may be modified and shared similarly to free software. Such works are called free cultural works and are defined by the free culture movement. See free media license for details.
Definitions and important differences
There are three major maintainers of freeness definitions and license lists:
- Free Software Foundation (FSF/GNU)
- Debian, maintaining Debian Free Software Guidelines (DFSG)
- Open Source Initiative (OSI), maintaining the open source definition
The criteria of acceptance are different. For example,
- Free Software Foundation does not require free software to contain only free media, while Libregamewiki does.
- Debian requires everything in its “main” repository to be free, including media.
- Some licenses are accepted by some and not others, e.g. Debian and OSI approved Artistic License 1.0, but FSF did not, while CC0 was approved by the FSF but not OSI.
Free Software Foundation
Software is free software if it grants everyone without discrimination four basic freedoms:
- Freedom 0: The freedom to run the program, for any purpose.
- Freedom 1: The freedom to study how the program works, and adapt it to your needs.
- Freedom 2: The freedom to redistribute copies so you can help your neighbor.
- Freedom 3: The freedom to improve the program, and release your improvements to the public, so that the whole community benefits.
Although it is not specified in those four lines, that includes for-profit performance of those activities; also, copyleft is allowed.
For the practical application of the definition, see FSF’s page Various Licenses and Comments about Them.