map lights are the lights located above to the front two or 3 feet forward, centered, right above the driver or passenger'* heads. they are called map lights because they shine right down into the lap of either seat where you would be holding a map.
the dome lights are in the middle of the vehicle generally and light up the 'whole' interior. this is so you can find something when it'* dark out. also many times they come on when you open a door...generally they have a switch for always on, always off, and on when door is ajar/open. problem is they blind the driver at night and scream pull me over officer i'm looking for my gun/drugs/beer. map/reading lights on the other hand generally do not, thus why we have them in our cars.
picture for an example:
the ones in the front are map lights then one in the back is a dome light.
this car for instance has 2 dome lights(note map lights are turned off):
the step/courtesy light is as follows(note this person is a ricer and made his blue which is gaudy, ugly, stupid, and provides horrible lighting):
i hope this clears things up for all of you. as i said above the names really give away their location/use...or a quick Google would give you the same info.
hope you're all enjoying the long weekend!
edit: since we are on the topic of overhead lighting and there seems to be some confusion let me go on. there is one more type of overhead lighting that comes to mind. these are called reading lights which are usually found above the passenger'* seats window to the left(if on driver'* side) or to the right(if on passenger'* side) and shine down similarly to map lights but from the sides not the center. Commonly found built into the 'oh ****' handle or the emergency handle as they are called. they are made NOT to blind the diver in any way shape or form but to allow reading at night for passengers in the car, as the name implies. if i find a pic i'll show you but as with all of this lighting it'* pretty self explanatory. Note not all cars have these.