First of all, it's ADVANCE WARS, not Advanced Wars.
It's a Strategy RPG along the lines of Tactics Ogre, Final Fantasy Tactics, Shining Force 2, or Fire Emblem. It's set in a modern time period, but in a fantasy world. Units include fighters, bombers, tanks, infantry, etc. The story is not very deep, but the gameplay is quite challenging, and the branching level design allows for a great amount of replay value. It's a very good game, and I would definately advise getting it.