Draw Something is probably the current most popular game app on the iOS and Google Play Store. It's more or less an online pictionary game, where people can login via their Facebook account and invite friends to play.
The aim is to pick a word and draw something relating to that word for your friends to guess. If they guess correctly then you both win a certain amount of coins based upon the difficulty of the word, and these coins can be spent on buying additional colours to draw with, or bombs, which can be used to eliminate letters if you're stuck trying to guess what word it is. As you guess more words correctly with a friend, the words get harder and harder.
The game is cross platform, meaning people on Android can play with their iOS friends and vice versa. The game comes in to flavours - a free version, and a paid for version that has 2,000 more words than the free version. For Android the full version is only £0.62.