This keeps coming up so I may as well try to help with the basics, I am not a lawyer but I’m familiar with copyleft.
The most successful and widely used copyleft license for software is the GNU General Public License also known as the GPL, the most successful copyleft license used for free knowledge is the GNU Free Documentation License also known as the FDL. The Linux kernel is licensed under the GPL, Wikipedia is licensed under the FDL. They are both GNU Licenses written by Richard Stallman in the early 80’s. Richard Stallman is sometimes credited with inventing copyleft and the first free public licences.
Copyleft is commonly implemented by a license and is applied to works such as computer software, documents, music, and art. Whereas copyright law, by default, automatically restricts the right to make and redistribute copies of an author’s work, a copyleft license uses copyright law to ensure that every person who receives a copy of a work has the same rights to study, use, modify, and also redistribute both the work, and derived versions of the work. Such licenses typically do so by requiring that the same license terms apply to all redistributed versions of the work
Copyleft is a concept and a play on words