It is a Chrome App so you'll have to play it in Google Chrome for now but it will be on Steam as well when it's released and pre-orders will get the game for Steam.
Here's a 'WTF Is...' from Totalbiscuit that gives a nice first impressions of the game. I picked it up after watching this video and it definitely has potential.
Finally, here's where to get the game. There is a free demo that you can play for 15 minutes so you can give it a try before you buy.