Featuring a limited lineup of fourteen characters - the lead characters of each game, generally - the beta will feature three-on-three battles in some selection of the game's battle arenas, and will also contain at least some of the final game's summon lineup.
This is going to be a closed beta, of course. Some folks should be receiving a code to get in already, it appears, though the criteria for getting one of the instant-in codes is naturally not public. There is a lottery to try to get a code, though, if you don't have one - all you need is a Square Enix account. I entered the lottery myself just a few minutes ago, and you can do the same at the links below.
In other news, it was also announced that Jecht, coming soon to the arcade version, will be part of the console release package on day one of next year's release.
Source: Gematsu, NA Beta Signup, EU Beta Signup