There are multiple kinds of licenses that software can be released under.
- GNU GPL is what GNU/Linux is released under. Essentially, you are free to do what you want with software released under this code, provided that if you release it to the public you must also release the source code. Source code does not have to be released if binaries to modifications are not released to the public. This license highly favors the end users.
- BSD License
- Apache License
- MIT License
- WTFPL - DO WHATEVER THE FUCK YOU WANT TO PUBLIC LICENSE. The name says it all.
- CC-BY-SA - Without the no commercial and no modifications additions, this is a very liberal license. Anyone can use or modify this work, as long as they let others use it and attribute the author. This license lends itself to art better than it does to software.