If you're referring to the actual process of installing apps over-the-air, see this article explaining it: http://www.readwriteweb.com/mobile/2010/12/apple-best-kept-s...

Weird that this hasn't been submitted before. The process (and purpose):

You create

1. One or several team groups

2. One or several user groups

You can then proceed to push pre-release builds to TestFlight and have specific groups of users (press, volunteers, friends, iPod/iPhone 3G/3GS/4) try the build, before it's sent to the App Store. Sending a release to the App Store before testing it with a larger group of users can be pretty catastrophic.

As a bonus, you basically drag and drop your .ipa(?) file to the browser, and TestFlight does the rest. The users can then proceed to get the build through TestFlight's HTML5 app.

Before, as I understand, the alternative was to send your app files to people and have them import it in iTunes.

It looks like all real information about what the service does is under the blog.. From what I can gather, it allows you to manage beta/prebeta (free) access to the apps you are currently developing for iOS.

Or, if you have an iPhone (or iPad or iPod touch) running iOS 4.0+, you can try it out by becoming a beta tester for my app, [Split Decision](http://testflightapp.com/join/d9566ad38cbafb7444b0cf6c8b4aa0...).