So far, it's just racing. No weapons or any of that fancy stuff. At the moment, you can choose from 3 different drivers (Mario, Luigi and Princess Peach) and 2 different maps (Mushroom Cup Round 1 and 2). There is no lap count, no timer and no finish. You can just race around forever and try to beat the others (it's not hard, especially if cut a few corners). The AI is rather crude, but seems to work ok for the time being.
You should hop on over and play Mario Kart JS for yourself.