The event is meant to be a "stress test" for the game, to see if it can handle an increased number of players. There will likely be bugs and glitches, but an open beta is a good way to get many of them ironed out before the actual launch.
So, how do you do it? It's quite an easy process outlined by Blizzard here:
- Log in or create a Battle.net account at http://www.battle.net
- Create your BattleTag™, which is required to play Diablo III (What is BattleTag?)
- Download the game client, install, and play between 12:01 p.m. PDT April 20 and 10:00 a.m. PDT April 23
The game client is about a 15 gig install, but as I write this, it seems to be moving fairly fast on my non-supercomputer. All five classes are playable, and though I'm not sure exactly how much gameplay there is, at least in the old beta it was a fairly substantial amount. I know at the very least you can level each character up to 13.
I was going to attempt to catch up on some reading this weekend, but when I got this in my inbox this morning, I knew my plans were about to drastically change direction. Download the beta for yourself, and I'll see you in hell until Monday morning.