Another Question

In Chapter 42, Abel Magwitch tells Pip about Compeyson. My question is, who exactly is Compeyson, and why does he want to catch Magwitch? I think that Compeyson is the other convict, but I'm not sure if I read the passage right. If so, is Compeyson's desire to catch Magwitch because of something that he did, or what happened? Please tell me if I am on the right track, or completely wrong.