Q: Is apple airplay an app?

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

Apple TV (2nd and 3rd gen)- How to use AirPlay Mirroring

How to set up and configure AirPort Express for AirPlay and iTunes

About AirPlay Mirroring in OS X Mountain Lion

iTunes 10- About playing music with AirPlay

Troubleshooting AirPlay and AirPlay Mirroring

Using AirPlay

Thanks to the $15 Beamer, AirPlay streaming is still possible on Macs  that do not support Airplay and mirroring.

Another solution is the Air Parrot.

Did you actually read the post? It's part of OS X on specific Mac models.

Airplay is a part of iOS, there is nothing to download.

If trying to use Airplay to send content from an iPad to an AppleTV, both devices must be on the same Wi-Fi network.

An Airplay icon should automatically appear to the right of your playback controls when your iPad detects an AppleTV on the same network. If it doesn't then first make sure that AIrplay is enabled in the AppleTV's settings, then try restarting the iPad, AppleTV, and wireless router.