After signing up, you will have to provide general information about your ISP, computer specs and your location. OnLive uses this information to organize beta testers into groups. If you fit into a particular test group, OnLive will email you and request that you run a detailed performance test on your network connection and system configuration. They will review that information and may choose your setup to install the OnLive browser plug-in.
For the unfamiliar, OnLive is an on-demand video game service which takes the computational load of running today's high end games off of your PC, and places it onto their servers. Games are synchronized, rendered, and stored remotely and delivered via broadband Internet. The service allows you to play games on low-end systems running Windows XP, Vista (and presumably 7), as well as any Intel-based Mac with OS X.
You're probably thinking that the service will host the typical lame browser-based games, but it has numerous big names on-board, including Electronic Arts, Take-Two, Ubisoft, Epic Games, Atari, Codemasters, THQ, Warner Bros., 2D Boy and Eidos Interactive. It has a library of top-end games you would expect to see on the shelves of your local GameStop.