When you rent a car you can choose between several different types of cars.  To try to make renting a car, or a type of car, easier, rental agencies generally break all cars down into five categories.  Each of these categories usually comes with a different series of rates, and although limos and vans are available, just about all other vehicles will be classified as one of the following: economy car, compact car, mid-sized/intermediate car, full sized car, and luxury car.

A car in the economy class would be a smaller two door car, like a Geo Metro.  These cars usually have room for two adults, two children, and two small suitcases.  These are also generally the cheapest.  The next step up would be to a compact car, which could be two doors, or four doors.  Two of the most popular rented compact cars are Ford Escorts and Chevy Cavaliers.  These are generally seen as having room for two adults, two children, one large and two small suitcases.

The next step is up to the mid sized, or intermediated sized, car.  These are two or four door cars that include cars like the Ford Tempo or Buick Skylark.  These vehicles have room for four adults, and several large suitcases.  The full sized car would be a large car like the Ford T-Bird or Buick Century.  These have room for four adults, plus plenty of leg room, and a lot of luggage.

Then there are the luxury cars.  These cars generally have four doors and include the Ford Crown, Lincoln Town Car, or Pontiac Bonneville.  These cars have room for five adults and four large suitcases, plus extra bags.  These are good vehicles for family vacations.  There are convertibles available, too, but they will obviously cost more and are not as good for family travel.
Most travelers will recommend at least a mid sized car, since these models offer the engine power and special features needed to tackle most terrains (such as mountain roads) while having safety features like airbags.  Think of what you need the rental for, where you will be going, what you need the car to provide reliably over the course of your trip.  Based on that, match your needs with the class of rental car and make your choice from there.  Especially in new places or long trips, having a car you are comfortable with is very important, and will help guarantee a safe and comfortable trip.