So you made a mod. Excellent, but even though you may think you are done, you still have one final step before you can distribute it to everyone: you must choose how it will be licensed. Licensing affects how others will be able to use your mod. Too strict and your mod becomes isolated and usable only for playing, too loose and you may find parts of your mod in the next major commercial game without a hint of crediting.
We are going to assume that you wish to have your mod available for free, and that others will be able to use your resources. Glest itself follows these limitations, so not doing the same is frowned upon and will likely reduce the success of your mod. For this purpose, there are three main licenses.
Primary types of licenses
The GPL, or GNU General Public License is a free license that gives the ability to let others modify your work, and is the most commonly used free license. The licenses for most software and other practical works are designed to take away your freedom to share and change the works. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change all versions of a program--to make sure it remains free software for all its users. Developers that use the GNU GPL protect your rights with two steps: (1) assert copyright on the software, and (2) offer you this License giving you legal permission to copy, distribute and/or modify it. To protect your rights, the GPL needs to prevent others from denying you these rights or asking you to surrender the rights. Therefore, you have certain responsibilities if you distribute copies of the software, or if you modify it: responsibilities to respect the freedom of others.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must pass on to the recipients the same freedoms that you received. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights. For the developers' and authors' protection, the GPL clearly explains that there is no warranty for this free software. For both users' and authors' sake, the GPL requires that modified versions be marked as changed, so that their problems will not be attributed erroneously to authors of previous versions.
CC-BY-SA / CC-BY-NC-SA
Creative Commons Sharealike and Non-commercial sharealike are both very similar. People can share - copy, distribute and transmit the work, and remix -adapt the work.
At the limitation that you must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the work). If you alter, transform, or build upon this work, you may distribute the resulting work only under the same or similar license to this one.
These limitations can be waived if you get permission from the copyright holder, and where the work or any of its elements is in the public domain under applicable law, that status is in no way affected by the license.
CC0 (Public Domain)
If you don't want to have any restrictions at all, and don't mind if others can use your work in any way (which can include uncredited usage in commercial applications), then CC0 (zero) is basically a public domain license.
In contrast to CC’s licenses that allow copyright holders to choose from a range of permissions while retaining their copyright, CC0 empowers yet another choice altogether – the choice to opt out of copyright and database protection, and the exclusive rights automatically granted to creators – the “no rights reserved” alternative to our licenses.