As said, most receivers which offer more than one zone provide the main zone as a surround area. That is, your main theater area with 5.1 or 7.1 audio setup. So, you put your 5 speakers + subwoofer (or 7 speakers & sub) into your room, and the receiver powers everything just fine.
The second zone may work any number of ways depending on the exact make/model you are looking at. Some receivers include 7 channels of amplification (7.1) and if you only use 5 channels (5.1) then they allow you to use the 'extra' two channels to power speakers in zone 2.
What is zone 2?
Zone 2 could be anything you want it to be... Have some speakers you want to put into your office? That's zone 2! Your bedroom has surround, but you want a couple of speakers in the bathroom? Zone 2!
There are more complex ways you can set up zone 2 so that you can run many rooms with the same audio. You would typically use an external amplifier and need to do some impedence matching, and put volume controls in all the rooms. It's more detailed, but if you are thinking of going this route, just ask for the details.
If you are running a 7.1 audio setup, and/or you don't have the amplification for zone 2, then you will need to get an external amplifier to run the speakers in zone 2. You can pick an external amplifier up for under 100 bucks via eBay.
What can you listen to in zone 2? The reason why it is called zone 2, is because it acts like a truly independent area of your system. Without the main surround area turned on at all, you should be able to turn on anything you want to listen to in zone 2. Listen to the radio, or a CD, or XM, or whatever you want. It does not have to be the same as what you listen to in the surround zone, and it doesn't have to be on when the surround zone is on.
Basically, take a stereo A/V receiver, and put all the same sources you have on your surround receiver into it, and that is what 'zone 2' is like.
Always, details help us understand what you want to do, but traditionally, more expensive receivers offer zone 2, and some offer zone 3, or even zone 4.
My home has about a dozen differernt zones which can each independently listen to music.