All GPS systems for the car operate with the same technology, employing a series of orbiting satellites to triangulate and locate positions on the Earth's surface with an excellent degree of accuracy. The difference between car systems is then a matter of the specific platform used to integrate the GPS system into the car. These break down into three primary platforms. Each has its own advantages and disadvantages. All three types do excellent jobs of navigation, so you won't suffer in performance when you choose any of them.
Factory-installed In-dash GPS Systems
A factory-installed GPS system is, as the name suggests, installed in the car when its built. The advantage of these systems are both cosmetic and functional. A GPS system designed to fit into the dash and styled to match the vehicle's interior can be an attractive yet unobtrusive addition to the car. Functionally these systems may also be integrated into the car's controls to a greater degree than systems that are added on by the consumer, allowing control of certain aspects of the system through the steering wheel controls, for example.
Aftermarket In-dash GPS Systems
Aftermarket in-dash GPS systems are basically aftermarket car radios with a GPS navigation system built in. You'll find in-dash aftermarket GPS receivers from almost all manufacturers of aftermarket car audio gear--Pioneer, Alpine, JVC and Clarion to name a few. Aftermarket in-dash systems have the advantage of options: you can buy a receiver that not only has GPS, but also includes a range of other audio features, as well as improved performance versus most factory stereos. But they do have the disadvantage of necessitating an installation into the car, plus the fact they'll probably never look quite as "polished" as a GPS system installed at the factory. Nonetheless, they offer excellent performance and great value.
Portable GPS Car Systems
Portable GPS car systems are increasingly popular. They're relatively inexpensive, mount easily onto your dash or windshield, and can be taken out of the car to prevent theft. Many are small enough to be used as hand-held portables when you're not driving, and feature extras like JPG picture viewers and MP3 players. Garmin's Nuvi series and GPS units made by TomTom are excellent examples of portable GPS car systems. They do have the disadvantage of small screens, and the need to plug in to your car's 12-volt power when the battery is drained. But if you're looking for the easiest way to add GPS to your car, they're the way to go.