Copyleft is a play on the word copyright and is the practice of using copyright law to remove restrictions on the distribution of copies and modified versions of a work for others and require the same freedoms be preserved in modified versions.
Simply put, traditional copyright law restricts the right to make and redistribute copies of a particular work, while copyleft licenses allow the free use of a work (including copying, modification, and redistribution) as long as the user does not restrict these rights in his copies and derivative works. Using a copyleft license is therefore different from releasing a work to the public domain.
Copyleft licenses are based on copyright law. Copylefted content is not free from copyrights. Anyone who wishes to reuse copylefted content legally must pay close attention to the specific terms of the license, and take certain steps described in the license.
Examples of copyleft licenses are:
The wiki-based online encyclopedia Wikipedia is fully under a copyleft license (originally the GFDL, then they migrated the license to CC-BY-SA). So is this site, the Spanking Art wiki, which missed the window for migration and is therefore still under the GFDL.