Copyleft gives anyone the right to copy, modify and / or distribute copyrighted material, subject to certain conditions. The most important requirement is that once something is copyleft, it stays copyleft. You may copy the original work, mass distribute it, or whatever, but you cannot remove the copyleft, even if you make modifications.
A copylefted work is governed by a licence. The first real copyleft licence is GNU’s General Public Licence, which despite its name is very software orientated. GNU, also known as The Free Software Foundation, is the philosophical home of copyleft. Visit the GNU site for a lot more material on the subject.
The GNU Licences are excellent for software, but not for more general use. With that in mind, I’m licensing my site using the Creative Commons Attribution NonCommercial ShareAlike Licence. See this site’s © & licence notice for more information.