Wikipedia provides interesting article about vehicle tracking.
Below is a small excerpt.
Several types of Vehicle Tracking devices exist. Typically they are classified as Passive and Active. Passive devices simply store GPS location, speed, heading and perhaps key on/off, door open/closed. Once the vehicle returns to a pre-determined point, the device is removed and the data downloaded to a computer for evaluation. Active devices also collect the same information but usually transmit the data in real-time via cellular or satellite networks to a computer or data center for evaluation. Examples of companies that offer Passive Devices are TrackStick and TravelEyes. Examples of companies that offer Active Devices are USAT Corp., LoJack, StreetEagle, and TrackYourTruck. Companies such as Fleetilla offer active devices suitable for fleet management applications. Online systems can provide information on demand or on a scheduled basis. Scheduled updates can be as frequent as once per minute.
