Copyleft refers to an approach taken by some FOSS licences, most notably the GPL, in which the Copyright holder grants users of his or her software rights to use or modify that software provided that the modified software is licensed under the same terms.

It is essentially a mechanism to ensure that the software product remains open source, by preventing people from taking the software and making it proprietary. Examples of licenses that do not adhere to the principle of copyleft are the so called "permissive" licenses, like the BSD license, that do allow the software to be taken and absorbed into a proprietary application.

Copyleft licenses are strongly supported by many proponents of FOSS (like Richard Stallman) as their "copyleftness" is what ensures that everyone benefits from every modification to the software.

It is important to note that Copyleft is not an "alternative" to copyright, nor is it the "reverse" - Copyleft is absolutely dependent on the existence of proper old fashioned copyright which reserves the right to the author of a piece of code to license it in whatever way they choose.

Copyright cannot simply be "given away" - it's an inherent right that a developer (or her employer) acquires the moment they create some code. Copyright can be passed to another person or organisation - but only by a formal agreement. This also means that the copyright holder is free to license code to which they own the copyright in any way (or any number of ways) they choose - So the creator of a groovy application can jointly license it under a Copyleft license, and under a proprietary license.