In addition to Roof Rack Systems, you can also carry your gear with rack systems that mount to a Trailer Hitch. But the first step would be to have a Trailer Hitch installed onto your vehicle. Fortunately there are Trailer Hitches available for most vehicles.
With a Trailer Hitch installed, you then have a choice of adding a hitch bike rack, hitch ski rack (requires bike rack), hitch cargo box, or hitch cargo basket. These rack systems slide into the receiver hole in the Trailer Hitch and are held in place with a bolt or cotter pin. Or add the appropriate towing drawbar, ball and wiring harness to allow you to tow trailers, campers, boats, etc.
Trailer Hitches are typically bolted to the chassis of the vehicle. Trailer Hitches are categorized by their load capacity into a few common classes: I, II, III, IV, and V.
The opening of the receiver-type hitches are usually 1.25" or 2" square, but some Class IV or V are available in 2.5" square.
When it comes to bike racks the most important thing to pay attention to is the carrying capacity of your vehicle's hitch, or the "tongue weight". This number, divided in half, gives you a good estimate of the maximum weight your hitch can carry (this number is very different from the maximum weight it can pull or tow)
For the most part, class I and class II hitches can only carry two, sometimes three, bikes. If you want to carry 4+ bikes, or have the ability to swing the rack to the side, you're going to need a class III or higher hitch.