Wow I just looked at this and it sounds really weird.
You basically paint the world your team's colour to win, but you have limited ammo so you have to turn into a squid occasionally on your own colour and you can crawl up walls and... Just what the hell is this?
(you can't just say 'made by nintendo = mario, shooting game = fortress' IMO ;P)
http://e3.nintendo.com/games/wiiu/splatoon/ is the link to the Nintendo @ E3 page that has all the game details on as well as the announcement trailer.