Sidescrolling jump & run. It kinda suffers from feature overload, your character can morph between large and small with the push of a button (think small/super Mario), attacking happens with weapons, one of which lets you pick enemies up and throw them like in SMB 2 or you can jump on enemies which makes them spin like a twister (this does damage to bosses BTW, you don't need to throw enemies at them although you can do that too) and when you jump off they slide around like the turtle shells in any SMB game, you collect money and buy items, you can select one out of two stages for level 2 and 3 and there's NPC houses where you can interact with people. Overall it's nice but I really don't know what half of those items do. You can get through the game without caring about them though.
EDIT: Forgot to mention the 2 player coop.