Copyleft is the generic name for licensing schemes that permit some free copying, adaptation or derivative work provided the copy is also issued with an identical licence.

Note that copyleft is not in any sense the opposite of copyright, and seldom means the same as 'public domain' (where no copyright exists). Copyleft licences actually  rely on copyright for their enforceable limits on what may be done. For instance copyleft usually forbids copying for commercial purposes and without attribution. Breaking the terms of a copyleft license is no different from any other infirnging use.

Copyleft licensing is most commonly applied to software, where Free Open Source Software has proved highly successful, typically using the GNU General Public Liicense. This allows software to be freely used,  improved and distributed by users but disallows commercial exploitation or rebranding. Revenues are not derived from sales but from indirect services such as installation, training, and consultancy.

There are attempts to adapt copyleft principles to writing and artistic works such as photographs, Creative Commons licenses being the best known. The intention is to encourage sharing without necessarily relinquishing economic rights or bylines. However a major problem is that rather often neither the terms nor consequences are well understood by photographers or photo users and clients. Creative Commons licences are widely interpreted as meaning the copyright user has little interest in asserting the terms so are relatively safe to steal. This perception is reinforced by an almost total lack of case law against infrinngers.