GNU is not in the public domain. Everyone will be permitted to modify and redistribute GNU, but no distributor will be allowed to restrict its further redistribution. That is to say, proprietary modifications will not be allowed. I want to make sure that all versions of GNU remain free.

the freedom to modify the work, and the freedom to distribute modified and therefore derivative works

The economic incentives to work on copyleft content can vary. Traditional copyright law is designed to promote progress by providing economic benefits to creators. When choosing to copyleft their work, content creators may seek complementary benefits like recognition from their peers.

Common practice for using copyleft is to codify the copying terms for a work with a license. Any such license typically includes all the provisions and principles of copyleft inside the license's terms. This includes the freedom to use the work, study the work, copy and share the work with others, modify the work, and distribute modified versions of that work.

Copyleft licenses necessarily make creative use of relevant rules and laws to enforce their provisions. For example, when using copyright law, those who contribute to a work under copyleft usually must gain, defer or assign copyright holder status. By submitting the copyright of their contributions under a copyleft license, they deliberately give up some of the rights that normally follow from copyright, including the right to be the unique distributor of copies of the work.

The strength of the copyleft license governing a work is determined by the extent its provisions can be imposed on all kinds of derived works. Thus, the term "weak copyleft" refers to licenses where not all derived works inherit the copyleft license; whether a derived work inherits or not often depends on how it was derived.

"Full" and "partial" copyleft relate to another issue. Full copyleft exists when all parts of a work (except the license itself) may only be modified and distributed under the terms of the work's copyleft license. Partial copyleft, by contrast, exempts some parts of the work from the copyleft provisions, permitting distribution of some modifications under terms other than the copyleft license, or in some other way does not impose all the principles of copylefting on the work. An example of partial copyleft is the GPL linking exception made for some software packages.

The "share-alike" condition in some licenses imposes the requirement that any freedom that is granted regarding the original work must be granted on exactly the same or compatible terms in any derived work.