That open beta will only run until 10 a.m. PT Monday morning. While you can't yet login to the servers, you can start downloading the client now, but be aware: the load is heavy, and the download speed is slow. You might be able to download the initial app installer quickly, but after that is installed it has to download the full client, which appears to be somewhere around 3GB in size.
Once they get the client fully installed, open beta players will then be able to try and work with their characters, which can come from any of the game's five classes, up to a limit, of level 13.
The five character classes are below. In past versions, classes were gender-locked, but in Diablo III, players will be able to choose a gender for any class character.
Witch Doctor: new, but similar to the Diablo II Necromancer. As evidenced by the name, though, his skills might be more traditionally associated with voodoo culture and shamanism. The witch doctor has the ability to summon monsters, cast curses, harvest souls, and hurl poisons and explosives at his enemies. As is normal for magically inclined character, mana is used to power spells the witch doctor casts. Mana must regenerate to be used.
Wizard: think Sorceress for Diablo II or sorcerer from Diablo. The wizard fuels his or her spells with arcane power, which is a fast regenerating resource pool.
Monk: is another melee attacker. In a way, he's similar to the assassin from the Diablo II expansion as he uses martial arts. However, also adds elements of the "holy warrior" known as the Paladin. Blizzard made it quite clear, though, that the Diablo III Monk is not in any way related to the Monk class from the Sierra Entertainment-made Diablo: Hellfire expansion. The Monk uses spirit as a resource pool.
Demon Hunter: here's your archer. The Demon Hunter combines elements of Diablo II's Amazon and Assassin classes. Crossbows as their main weapon, but they also hurl bombs at enemies. The Demon Hunter is uses discipline and hatred as resource pools. Hatred is a fast regenerating resource that is used for attacks; Discipline is a slow regenerating resource used for defense.
Closed beta players will continue to be able to play this weekend, and afterwards, at least until the beta completely shuts down on May 1. Then, it will be just two weeks until the official May 15 release, and Blizzard servers that will probably be hammered.
Diablo III comes in two flavors: a standard edition that retails for $59.99, and a Collector's Edition.
We wouldn't be surprised to see game patches released before or shortly after the Diablo III release; nowadays, it's almost typical.