the purpose of an amp is to amplify a signal sent to it. a reciever is a combination of several different components. there is the pre-amp which does any kind of processing of a signal provided by cd player, dvd player, etc. there is the amp that then takes the processed signals and amplifies them and sends electrical signals to the appropriate speakers. there is the tuner which lets you pick up radio waves. there is also the power supply. reason for buying seperate components is that usually to keep prices reasonable most companies will not use the best things on their shelves for power supplies. the most power hungry part would be the amp. also having separate power supplies for all the components is supposed to make the individual parts function better. i cant remember the reasoning but am sure more knowledgable members can tell you why.