(I)ntelligent (D)ance (M)usic.
It's one of my least favourite genre names.
Firstly because it implies that all other "dance" music isn't intelligent, which is just patent bollocks, and secondly because a lot of "IDM" isn't particularly danceable anyway.
I think Electronica is a better word for it.
What is it?
It's basically a catch all term for electronic music that doesn't fit into any other pre-defined slots.
Oooh, also just remembered ISAN: http://www.isan.co.uk/