What is the illuminati?

I don't really know what is an illuminati. The only place I've hear of it was in angels and demons, the movie. But.. yeah i know its related to something new world order. i heard they're pretty powerful. Someone say the swine flu was caused by them, to lower our population? i heard they were the ones who killed Micahel Jackson, marilyn monroe, john f.kenedy, and 2pac. Yeah.. is Jay z invovled with these things? yeah you get my question..