While the rental car companies all use different vehicles and different models, the cars available all fall in similar ranges. At the lowest priced end you have Economy cars and compact cars. Then the grouping of Mid size cars, Full size cars. These will generally be split into 2 door models and 4 door models. Then you have Premium and Luxury models. They will be priced in the same range as Minivans, convertibles and Sport Utility Vehicles if the company you choose has such models. Then there are special circumstance choices like a 15 passenger van or a PT Cruiser or a Jaguar.
Alamo has the following information on their site and it is the best summary out there: