Developer Bungie released a video hosted by celebrity comedienne Aisha Tyler today to promote the event. She goes over all the new features and game modes. Watch one minute of it and you'll find yourself very conscious of the target demographic — teenage and early 20-something males.
The beta began today and runs up through May 19. Bungie expects as many as three million players to participate. The term "social gaming" seems to be an understatement when we're talking about a group larger than the population of most major North American and European cities.
If you'd like to be among the three million, read Bungie's instructions for getting started over at the developer's website.
Note that the beta test is multi-player only, so you won't be playing through the campaign. You'll just be killing your friends repeatedly in several new gameplay modes, including Headhunter, Arena, Stockpile, Invasion and Generator Defense. Some of those are variations on existing modes — for example, Stockpile is a multi-flag version of capture the flag that requires players to hold captured flags for a period of time in order to score. Have fun, gamers.
Halo: Reach Beta Introduction by Aisha Tyler