WoT is a free to play/pay to win (depending on your feelings) tank combat game. About 15 players per side are placed in a big battlefield and you have to either eliminate the enemy team or stay in the capture point of their flag for 60 seconds. There are tanks from the USA, France, Germany and the USSR (pre and during WW2 era) and you follow a tech tree of upgrades. You start with crappy light tanks and can work your way up to medium and heavy tanks or instead drive a self-propelled artillery gun or a tank destroyer.
Because it's free and each match usually lasts about five minutes it's a great quick game to play.
I had an account on the EU servers but they changed the tech tree around from the beta so the tank I really wanted (Pzkpfw IV) was moved and I wasted hours working towards where it used to be! So I might as well make a new account on the US servers I guess.
It's not for everyone, a lot of the early tanks are very weak and slow (especially the French) but it's still lots of fun. It also took up 16Gb of my hard drive which I couldn't spare when Borderlands 2 was coming out...