We recommend having a quick read of the Airfoil manual - you can always access it from the Help menu.
Airfoil
Airfoil lets you send audio from almost any application to your AirPort Express and Apple TV units, as well as to other computers on your network. Airfoil also features two complimentary applications, Airfoil Speakers and Airfoil Video Player, to enhance your experience.
Airfoil Speakers - Send audio to Macs, PCs, Linux machines, and iPhones and iPods Touch!
The Airfoil Speakers application makes it easy to stream audio to all sorts of other devices on your local network - Macs, Windows PCs, and Linux machines, as well iPhones and iPods Touch. Just run the Airfoil Speakers on any machine, and that device will appear as a set of remote speakers in Airfoil's main window, letting you can send audio to it.
You can download complimentary copies of Airfoil Speakers for Mac, WIndows, and Linux right from the Airfoil site, as well as Airfoil Speakers Touch from the App Store, and run them on any machine on your local network.

For more details, see Using Airfoil Speakers in the Airfoil manual.
Airfoil Video Player - Watch local video with remote audio
Airfoil Video Player enables you to play local video with remote audio, in sync. You can find Airfoil Video Player right in the Source pop-up, at the bottom of the main Airfoil window. Play your video with Airfoil Video Player, and Airfoil will handle sending audio to your selected remote speakers, in sync with the video.
For more details, see Using Airfoil Video Player in the Airfoil manual.
A Friendly Reminder
Airfoil is currently in trial mode and the quality of all transmitted audio will be degraded after ten minutes. For full-quality audio, please purchase a license.