Ratings are from CruiseCritic.com and indicate the percentage of reviewers who "loved it", meaning their experience on the ship. (Percentages, rankings, and information updated Jan 2019.)
According to websites such as CruiseMates, large ship lines are divided into somewhat subjective categories. These include:
Mainstream or Contemporary (Carnival, Norwegian, Royal Caribbean, Costa, MSC, Thomson)
Premium (Celebrity, Disney, Holland America, P&O, Princess)
Deluxe (Azamara, Oceania, and probably Cunard)
Luxury (Crystal Cruises, Regent, Seabourn, Silversea,)
A notable difference between Deluxe and Luxury is usually that alcohol is included on a Luxury line. Shore excursions may also be included on Luxury lines.
There is also a "Budget" category but apparently there are no established lines in the category at the moment. There is a former European line, EasyCruise, that had passengers buy their own meals, including sandwiches or room service. Most hotel services were also extra. EasyCruise was considered a budget line. A mainstream line would include meals (except some premium restaurants), room service (usually), and entertainment.
There's also an "Expedition/Adventure" category although those aren't usually large ships. Instead, they may be engineered with additional stabilizers for rougher waters (think Antarctica) or reinforced hulls in case there's need to act as an icebreaker.