power supply

That's kind of a ambiguous question.  Are you talking something like an ATX power supply or something different?  In the most basic terms the main parts are usually a rectifier, power factor correction (PFC), power controllers, mosfets, and the associated inductors, caps and fuses.

Take a look at the Digi-Key Reference Design Library to see some examples. 

http://designs.digikey.com/library/search/4294959904/4294959862#N=4294959862+4294959904