This article on parallel universes is linked on a few sites I have previously listed, however I believe it is worthy of a direct link. Tegmark provides a good, uncomplicated overview of the different types, or levels, of possible parallel universes. Some or all of them may actually exist.
The first three levels of parallel universe are completely distinct from our universe, and get progressively strange (for lack of a better description) comparison to the universe we are in. For instance, level one is essentially just regions of our present universe that are beyond our light horizon. It is estimated that the region of our universe we can actually see is only a very small portion of the whole, however the actual size and shape of our universe is not established. Level two parallel universes represent other post-inflation bubbles; in other words other universes that may have resulted from the big bang and subsequent inflation. In these universes the fundamental physics would be the same but the constants would be different. Finally, the level three universes would have different fundamental physics.
Level four universes can be considered somewhat distinct from the first three, in that if the parallel universes of quantum mechanics exist they overlap and interact in a way the other levels do not. These universes would all share the laws of quantum mechanics.