Copyleft is a form of licensing that can apply to computer software as well as a wide range of works including scientific discoveries, art, documents, etc. Copyleft licenses for computer software provide a reciprocal arrangement with users of free and open source software. If a developer makes any modifications to this kind of software and then distributes that derivative version, they must make their version of the product freely available with the same freedoms they received, allowing users to reproduce, adapt and distribute it.
The basic principle is that if you freely benefit from the work of others, you must likewise freely grant others the same benefits. The GNU General Public License (GPL) was written by Richard Stallman and became the first copyleft license specifically written for software to gain widespread acceptance, and is the premier example of software copyleft licenses today.