If you go to usacycling.org you can find all the info you need.
Cat 5 does not necessarilly mean less fit riders. A pro mtb racer that wants to dabble in road racing has to start off in cat 5. 10 races is the minimum requirement to 'cat up' to cat 4. It is very difficult to become a cat 1 and most people can't do it. It takes full dedication and a lot of talent. Again a cat 3 could be just as good as a cat 1 but just has not gotten there yet and likewise a cat 3 could be just as bad as a good cat 5. Some people just want to cat up for the sake of being in a lower category but you really need to get some wins or you'll have a hard time and probably not enjoy it.