A receiver is basically an amplifier (stereo or surround) with in-built radio tuner.
You can connect your DVD player to a receivier for sound output.
A DVD player plays DVDs/ CDs and other formats like AVI, MP3 etc. They do not have power amplifier.
So you cannot enjoy home theatre sound without connecting this to a receiver.
But many times DVD players include an in-built amplifier and radio tuner, these are often found in HTiB (Home Theatre in a Box) systems.
Hi In addition to this you can connect multiple sources into AVR like
DVD player's Audio and video to AVR for processing the sound and Video.
STB(set top box like Dish tv,tata sky,Cable STB) audio and video.
PC(Personnel Computer) into AVR
Game consoles like(Play station-3)
This means all sources into AVR and there will be only one output into TV for Video and one Sound output to your Speakers.
AVR Will process the Video/Audio signal from all sources and deliver the output.