In effect there are three main types of carrier: the rear rack which clips around your boot lid and rests on the rear of your car; the roof rack which attaches to roof bars; and the towpoint mounted rack which can either fit on the towbar or towball.
If you don’t already have roof bars, they are easy to fit and can be bought for around £100 / $150. A towpoint, on the other hand, is a garage-fitted option and will cost significantly more.
You'll also need to consider how many bikes you are planning to transport. Roof racks are great for quickly loading one or two bikes or, if you put a little care into arranging them, even a full complement of four cycles. If you’re a little challenged in the height department, however, they can be a difficult to reach.
Rear racks can be loaded more easily but face other pitfalls: often the bikes have to be packed together so tightly that rub damage is a common occurrence unless you're very careful to wrap and isolate the tubes, and fully-loaded rear racks have a tendency to obscure your number plate and lights, making your car illegal.