What does it do?
Put simply, xAP provides the glue to interconnect different applications and devices within the home. It's an open standard, so anyone can create a xAP aware applications or devices. Any xAP device can potentially communicate with any other xAP device with the minimum of fuss, allowing you to, say, control your heating from a Slim MP3 player or display the name of a person calling your home phone on your TV screen.
As a good starting point, we'd recommend first installing some xAP applications on a PC to get a feel for what you can do.
For a quick overview on installing some xAP applications on a PC, see Stuart Booth's Tutorial: Getting Started Tutorial
If you are going to install more than one xAP application on a PC, you first need to install a hub. Install a Hub