What is copyleft?

I just noticed a curious note at the bottom of my legislator's web page: This site is "copyleft". You are encouraged to reuse any material on this site.

Now what does that mean? Of course, I found the answer in Wikipedia. Although the entry appears to have been written by a lawyer, I gather that copyleft is a form of copyright that allows free distribution and even adaptation of a work, while preserving at least some rights of the original author.

Just thought you might find that interesting.