Copyleft - All Wrongs Reversed
Copyleft is a general method for making a program (or other work) free, and requiring all modified and extended versions of the program to be free as well.
Common practice for using copyleft is to codify the copying terms for a work with a license. Any such license typically gives each person possessing a copy of the work the same freedoms as the author, including (from the Free Software Definition):
0. The freedom to use the work.
1. The freedom to study the work.
2. The freedom to copy and share the work with others.
3. The freedom to modify the work, and the freedom to distribute modified and therefore derivative works.