Meat Boy is a boy w/o skin. He looks about like a cube of stew meat with arms and legs. His girlfriend, Bandage Girl is kidnapped by Dr. Fetus--who is actually a fetus, in a jar... w/ a monacle--and Meat Boy must track her down, presumably so they can combine and form some sort of cubic mummy of sorts.
This sort of excuse plot harkens back to the classic platformers of the early days of NES (I keep thinking about plumbers for some reason) and the similarities w/ those classic games don't end there. The controls are simple and responsive but the level of challenge is at times harrowing. Split second timing is the norm after the first few levels and many levels seem impossible when first encountered. They are all beatable after some practice though and the feeling of accomplishment is about the same as that first time you knocked a giant dragon turtle into a river of lava.
The level design and progression are really what make Super Meat Boy shine. New and more difficult obstacles are introduced slowly and in such a way that the correct path though each level is either implied or where finding the correct path is the challenge itself. What works exceedingly well about Super Meat Boy--and indeed makes it more enjoyable than all but a very select few of classic platformers--is that each challenge is a short, self contained level. You have infinite lives and each time you die, you immediately restart each level, which reduces the frustration factor significantly. There is no playing for forty minutes and dying on an end boss only to have to play for another forty minutes to even try again. You play for at most a few minutes at a time so when you lose, no big loss (well, except to your pride). Still, some levels are going to take dozens or even hundreds of tries. That Super Meat Boy manages to put you through this and keeps you coming back for more is a testament to just how fun it is.
True, there's going to be very few players who manage to get through this w/o chucking their controller through something at some point but most will be laughing right as they curse. Some of the obstacles Meat Boy encounters are just downright ridiculous and this is a game that truly revels in its sadism. The crazy thing is though, that's just why it's fun.