Essentially it is to AE what AE is to regular FTL. It is a huge compilations of a bunch of community made mods in one extremely well made package. It includes a ton of new events and weapons, stuff to do in empty jump gates, a system for buying and selling goods for extra scrap and a whole bunch of other stuff I haven't encountered yet. As far as I can tell all the new content is pretty balanced and fits into the game. As someone who had found the game to be getting kind of stale it tottally revived my interest. You guys should totally check it out so if only to get some strategy discussion on the new stuff.
You can download it here.