Which Version of Air Display?
Air Display 3 is the latest and greatest.
Currently only for iOS 7 & 8.
Requires free host software on Mac 10.8 OS X and up (but the latest Mac host can still connect to older clients on iOS, Android, Mac, and Windows.)
Air Display 2 is available for iOS, Android, Mac, and Windows.
Connects to host on Windows as well as Mac.
The Original Air Display.
Although no longer available for purchase, it is still supported.