AirPlay was introduced with iTunes 10. It's really just an update to the functionality of AirTunes, which lets you share speakers over wifi.
However, there are a few new features that make it more useful than AirTunes. One of those is the ability to control the output of two separate speakers. In the past, (with AirTunes) changing the volume on one device affected the Master volume, because they did not have independent controls.
When you launch iTunes 10, you will notice the AirPlay logo in the bottom right corner of the iTunes window (the blue, hourglass-shaped icon in the picture below). Click on it to bring up devices that are compatible and connected on the same wifi network. In our case, that's an Apple TV:
If you select Multiple Speakers you can turn on both devices and use the check box to select devices you want to share the audio with. There is now a master volume and independent volume controls for each connected device. In the image below, it's only two.