The GNU General Public License (GPL) is one of the most important documents ever written on a computer. It is to Richard Stallman as the Ten Commandments is to Moses. Any software released under this license respects your freedom under strong copyleft, though it is not a guarantee the software itself is any good.
The four essential freedoms
According to gnu.org, a program is free software if the program's users have the four essential freedoms:
- The freedom to run the program, for any purpose (freedom 0).
- The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
- The freedom to redistribute copies so you can help your neighbor (freedom 2).
- The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this.