GNU License

"Many programs are distributed under standard licenses. The advantage is that everybody knows what is and is not permitted, and after wide usage, the wording is likely to be accurate and legally binding. Using a standard license saves time and effort in producing one’s own license, although it may not be exactly what the author wants. Most standard licenses allow third parties to sell the software. The GNU General Public License is the original and best known. Software may be used by anyone for commercial and non-commercial purposes and may be redistributed without restrictions, conditional on including the source code. The GPL permits the creation of derived works, but only if the derived work is also licensed under the GPL. This prevents GPL-licensed software from being transformed into a proprietary product with secret source code. Examples include Linux, MySQL, the GCC compiler, the EMACS editor and hundreds of other programs."