About AirPlay and Airplay Mirroring
AirPlay Mirroring requires a second-generation Apple TV or later, and is supported on the following Mac models: iMac (Mid 2011 or newer), Mac mini (Mid 2011 or newer), MacBook Air (Mid 2011 or newer), and MacBook Pro (Early 2011 or newer). For non-qualifying Macs you can try using Air Parrot.
Several Apple Articles Regarding AirPlay
AirPlay is also used in some speaker and stereo system to send audio over a WiFI network. The Apple TV supports audio and video streaming. iOS devices can only send audio to one speaker.
When used with iTunes on a computer you can send audio to multiple devices at the same time for a whole home audio system. This setup can can be quite handy when controlled on an iOS device using Apple's free Remote app.