It's 2 teams consisting of maximum of 5 heroes battling it out.
The map has 3 lanes which act as the pathway to the bases. Every 30 seconds a wave of creeps from each end of every lane spawns, that means 3 waves from each team. The creeps move along the lanes, attacking any enemies they encounter, until they reach the enemy base or are killed.
Each lane has 3 towers at certain intervals which kill off the creeps, slowing down all attacks from that lane. The nearby area is thus the best place to defend for incoming attacks. The towers also detect invisible heroes and units, which there are a lot of.
You cannot destroy the towers in the back until you have destroyed every tower before it. This is so that powerful heroes can't just walk into enemy base and start destroying it when no one is looking, although after enough towers are down this too is possible.
The most basic premise is that you are in groups of 2, 2 and 1 spread on each of the 3 lanes defending them. You level up if you are near enough to dying creeps and gain gold if you manage to last hit them. You also gain 1 gold per second no matter what.
As opposed to defending a lane, you can also go to the woods and "jungle", which means killing off neutral creep camps for gold and exp. This only works on very few heroes at the start of the game but because there is no one else there to last hit the creeps you gain gold faster and the exp isn't shared with anyone else either. Basically this is the WC3 ladder equivalent of creeping. The neutral creeps do not drop any items though, nor do the team specific creeps.
You can buy items, and recipes, which combine lot of items into one, sometimes giving them special features. All the recipes are predefined there is no dunamic item building or whatever, but the selection is big enough to confuse any new players.
The amount of heroes is also something that will confuse new players and makes the learning curve a 89 degree wall. If you don't know what the heroes in your team, in the enemy team and the one you have do or work then you are at a BIG disadvantage. Apparently there is a total of 93 heroes currently, according to the wiki entry.
The level cap for all heroes is 25.
There is a special boss creep who drops items and gives gold for the whole team when killed.
And that's about it I guess?
EDIT:
PS I can play anytime now up until sunday night pretty much