I noticed there's quite a bit of tekkies here on the forum, so I thought I'd ask to see if anyone could tell me about Joomla! - I was told it's becoming the next big thing in content management and that knowing it would be a huge plus on my resume.

So, in a nutshell, what is it? what are the basics I should learn? Why is it such a big deal?

PS - I downloaded it thinking it was an application, but it looks like a site(?). Is it something I can run on a mac?