GNU Free Documentation License

To quote the license itself:

"The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others."

Here's where you can read the whole thing: http://www.gnu.org/copyleft/fdl.html