"A barebone computer is a half-assembled platform that consists of a case with a power supply, a motherboard preinstalled into the system case, and a cooling system." - From wikipedia
Rove.
Not ALWAYS the case...
A lot of times when you buy barebone, I believe the components come packed separately. I think the only things that HAVE to be included in a barebone is the motherboard and the case, but often you will see ones with PSUs, RAM, CPU or other components.