reciever vs seperates

Can someone either explain to me or point out a link that analyzes the differences between recievers and seperates? Specifically I know that seperates consist of pre-amps, amps, dacs, surround sound processors; what do each of these do? And more importantly which is likely to improve the sound over a standard reciever?

For example, I have a denon 3803 and i've heard that they don't have good dacs. Can I add a dac into this configuration and would it make a significant difference in quality. Which seperate is likely to increase quality the most over a reciever? thanks