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http://www.apple.com/osx/whats-new/features.html#airplay

you must have one of the macs in the article that GGJstudios linked to. basically within about 2 years old.

If you are planning on watching video using this, the quality will be lower and the video will probably stutter a bit.
Wired ethernet or fast wi-fi (802.11n 5GHz) will eliminate some but not all of this.

when it comes out in a few months OS X Mavericks will include the option to use airplay as a second display instead of just showing the same image as on the main display. It will probably have the same quality issues.
 
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