It's pretty simple. It's a PC that's hooked up to a TV. Usually, an HTPC is focused on media storage and quiet operation. You can really do it with any PC though. It's not like you actually have to have special parts for it...
You just hook up the audio the receiver just as you would any other source.
Depending on the uses, you can use either a wireless keyboard/mouse or have it all running off a remote.
The benefits of an HTPC is essentially the same as having a DVR, with the addition of being able to play downloaded content and streamed media.