None really, they mean the same thing. A minor point to note is that "someone" is usually (not always) preferred in a more "formal" context. So perhaps the way to think about it is that you use "someone" when you write, and either "someone/somebody" when you speak to friends... but there is no hard and fast rule on this word. When in doubt, I suggest you use "someone"?

The two words have the exact same meaning. Stylistically, "someone" sounds a bit more formal than "somebody" -- at least to me. The same is true of the other pairs: anyone, anybody; everyone/everybody; no one; nobody