What is it for a game?
Hard to describe - there was a massive discussion on the HoN forums where people argued how it should be classified. It's basically Warcraft III (for which the original Defense of the Ancients was designed) but played with no base-building and each player controlling a single hero. Instead of strategy you concentrate (even) more on tactics and your individual hero's skills and items. There are automatically spawning armies for both sides, locked in a stalemate battle unless the heroes influence it and the final goal is to lead them to destroy the enemy's base (plus kill the other team's heroes a lot ;-).
how long is the beta open for? if it's for more than just a few days left to go, I'd be happy to take a spare invite.
Couldn't find any exact dates but I guess it will be running "till it's done". It's certainly more than a couple of days since it's been active for weeks now with thousands of players online at the same time.
Ah - and if you like the game and don't feel like buying it later you can pretty much go back directly to Defense of the Ancients provided you have Warcraft III + Frozen Throne and can stand the pro-elitist community surrounding it. ;-)