There are basically 3 rack options:
- Roof mounted
- Trunk mounted
- Trailer hitch mounted
In the "trunk mounted" category there are those designed for conventional trunks and those designed for hatchbacks.
In all three categories there are racks that support the bike with both wheels on and "fork mount" racks that connect to the front fork to provide stability. Fork mount is especially handy for rooftop racks since it eliminates the need for some sort of vertical support which is unstable and hard to attach. But fork mount only works if you have a quick release on the front wheel and nothing such as a front rack that interferes with mounting.
There are arguments for/against all styles. The rooftop units require some strength and a bit of "tallness" to use, but are otherwise the most flexible, and capable of carrying 4-5 bikes. They do produce the most wind drag (and lost MPG) on long trips, though.
It may be hard to find a trunk-mount unit that suits your car. And a trunk mount unit is generally only well-suited to a single bike, maybe 2.
Trailer hitch units are pretty good -- easy to get the bike on/off, able to handle 2-3 bikes and not too sensitive to bike style. But they are not particularly stable on rough roads, and they of course require a car with a (fairly heavy-duty) trailer hitch.
(If you decide to get a trailer hitch installed to accept a bike rack, try to get a 2-inch "class 3" one, rather than the smaller class 2. This improves the stability of the rack.)