I have a somewhat embarrassing admission: I've been working in (or at least near) open source software for a little more than two years and, before All Things Open last month, I didn't really understood the subtleties around different types of open source licenses.
I wasn't a total IP ignoramous. I even took a graduate-level class in media law while I was in journalism school and we covered copyright in great detail. So, I understood that according to US law, everything was copyrighted upon creation (or more accurately, when fixed in a set medium). And I understood that there were open source licenses that essentially released works from copyright.
But (here's the admission) I thought "copyleft" was a specific license. I learned otherwise when Andrew Hall, a lawyer with Fenwick and West, LLP, who specializes in IP law, explained in his Open Source Licensing & Business Models talk that copyleft is not a specific license, but rather a term that describes all licenses that allow derivative works, but require derivates (if distributed) to use the same license as the original work.