Copyleft
Copyleft is what helps maintain free use of this software among other programmers. Copyleft gives the legal right to everyone to use, edit, and redistribute programs or program's code as long as the distribution terms do not change. As a result, any user who obtains the software legally has the same freedoms as the rest of its users do.
The GNU Project and the FSF sometimes differentiate between "strong" and "weak" copyleft. "Weak" copyleft programs typically allow distributors to link them together with non-free programs, while "strong" copyleft strictly forbids this practice. Most of the GNU Project's output is released under a strong copyleft, although some is released under a weak copyleft or a permissive free software licence.
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