In order to take your bike with you on your trip around town or around the country, you need a bike rack for your vehicle. There are 3 main types of vehicle bike rack, and understanding the uses and beneifts of each type will help you make a worthwhile purchase. Here's the skinny on each type of vehicle bike rack:
Hitch-mounted. As the name suggests, this type is attached to your trailer hitch. Hitch-mounted racks are the most popular option because you won't have to lift your bike over your head, and you won't experience the wind drag of having the bike on the roof of your car. The rack itself doesn't adhere to the body of your car, so your paint job will stay intact, too. These can be difficult to install, however, so shop at a place that offers to install it for you. The disadvantages of a hitch-mounted rack are decreased visibility behind the driver and the possibility of bike damage during a rear-end impact.
Roof-mounted. If your vehicle has roof rails, a roof-mounted rack is also an option, though these work best with lower cars. This is the preferred rack for those who want to minimize the risk of accident-related bicycle damage as much as possible. A roof-mounted rack also gives the driver unobstructed sight while driving. The disadvantages are that you'll have to get your bike(s) onto the roof of the car (if you're considering a roof-mounted rack, try lifting your bike above your head to make sure this is a realistic idea), and you'll experience drag while driving. Also, be sure to put a post-it note somewhere on your dash reminding yourself that your bike is on the roof before you pull into the garage.
Trunk-mounted. If you only need to transport your bike occasionally, this is a good entry-level option. Trunk-mounted racks attach to the back of your vehicle and hold the bike against your door of your trunk. These are by far the least expensive option, and typically easy to load. However, installation quality is the key to success with a trunk-mounted rack, so make sure you have done a quality setup. Also, trunk-mounted racks do not typically lock, so you should plan to not leave the vehicle unattended when the bikes are loaded. Similar to the hitch-mounted rack, driver visibility will be decreased in the rear, and there is a chance of bicycle damage in the event of a rear-impact collision.