From IFWiki

The GNU General Public License, or GPL for short, is the most common free software license. Originally written for the GNU operating system and later adopted by the Linux kernel and tens of thousands of other free software projects, it is designed to protect the following freedoms:

  • The freedom to run the program, for any purpose.
  • The freedom to study how the program works, and adapt it to your needs. Access to the source code is a precondition for this.
  • The freedom to redistribute copies so you can help your neighbor.
  • The freedom to improve the program, and release your improvements to the public, so that the whole community benefits. Access to the source code is a precondition for this.

A vital clause in the GPL is that all software that uses the source of GPL'ed software must in turn be released under the GPL. Thus, some of the freedoms listed above are only available to you if you are willing to extend them to others.

For a work of interactive fiction, this means that if you create a game that contains source code from a game released under the GPL, you must release your own game under the GPL as well. (Unless you obtain permission from the original author to distribute it under another license.)

Version 2

As of this writing (April 2007), the most current version of the GPL is version 2. Its full text can be found on the GNU website [1].

Version 3

As of this writing (April 2007), the third draft of the GPL version 3 has been released. The final version is assumed to be released later in 2007. This new version of the GPL has been written to protect users against new ways of limiting their freedoms--in particular through Digital Restrictions Management (DRM) and software patents.

Version 3 is now (September of 2008) final [2].