There are two main types of rear racks: trunk-mounted and hitch-mounted. The trunk-mounted rack is attached to the car with straps and the hitch-mounted rack is attached to the car by way of a pre-installed receiver hitch. Trunk-mounted racks are available for two, three, and four bikes. Hitch-mounted are available for two, three, four, and five bikes.
Roof racks offer the kind of versatility rear racks do not since they can carry many different combinations of items - a bicycle and a kayak, a bicycle and a storage compartment, multiple bicycles, etc. Roof racks are available for vehicles with and without factory racks, and can comfortably carry up to five bicycles depending on the vehicle's model.