It's important to know if your vehicle has gutters or not. Roof racks can be noisy from wind resistance. They require someone with strength to hoist the bicycles to the roof. You have to be careful not to forget that the bicycles are up there and drive into a garage. With some roof racks, you can't open your vehicle's sun roof, however, they do allow full access to your trunk.
Rear racks mount on the back of a vehicle with brackets and straps.
They can scratch paint and can be hard to attach. Most limit your access to the trunk, but they are generally inexpensive, and you can load bikes quite easily. Hitch racks mount on the rear of a vehicle but use a trailer hitch as their main point of attachment. They're less likely to scratch your vehicle but are more expensive. Sport trailers are good for carrying many bicycles, but remember that you'll pay extra if you drive on a toll road.