OMG, you don't know what Paper Mario is?
You have the power of the internrt, and you haven't found out what Paper Mario is?
Oh well.
It's a 2D RPG, where Mario, and all the characters are essentially 'paper cut-outs', in that when Mario turns around, he flips like a piece of paper. And changing rooms, levels, etc, makes the screen 'turn a page'. The environments, though, are modelled in 3D. Its like a mix of cel-shading and 3D.