Somewhere else here, I posted a breakdown I've seen on CC in the past - it's a lot subjective, but relates to what is included in your fare and things like passenger/staff ratio and passenger/space ratios. Usually newer ships within one company could be rated "higher" than older ships - but if the overall amenities are still the same, it doesn't usually move them from one market to another....
Luxury: Silversea, Seabourn, Regent, Crystal
Premium: Oceania, Azamara, Cunard, maybe Viking Ocean?
Mass Market: Princess, NCL, RCCI, Carnival, Disney, HAL
Lines like Sea Dream, Windstar - I find very hard to place, they seem to be very niche-market. For instance, my dad "hates" cruising but loves Windstar. I don't want to sail it because it doesn't offer balconies. Go figure ;-)
Cruise Critic is mainly known as a forum/message board, but they also have extensive member reviews, and they have an editorial board that posts reviews as well as articles - on things like "best line for XXX" - families, romance, food etc.