Monster Lab falls somewhere between an RPG and a beat em' up, where you piece together your own monsters under the supervision of your mentor, Professor Fuseless. He sends you on missions to help people and along the way you'll bump into rogue monsters to fight one on one.
It will take you a while to buy into this idea. At first Monster Lab looks like a dodgy beat 'em up for kids, but once Prof. Fuseless has shown you all the monster-building equipment in his workshop and you've had a few rounds in the ring, the explore/fight set-up actually works rather well. It's also quite a complicated game at first (which may have dented its appeal), what with all the different body parts to grow and the ingredients you'll have to combine to make your monster as strong as possible.