I have a couple of small quibbles with gerg's post:- when he says flight levels start at FL18 he really meant FL180 (approx. 18000 feet)- but, while that is the case in US airspace, it is by no means universal. Here in UK the Transition Altitude (the level at which you switch between altitude above mean sea level and Flight Levels) is between 3000ft and 6000ft depending on where you are.
What's the logic behind that? Or did I ask a trick question?