First and foremost there are two different types of trackers, you have active GPS trackers and passive GPS trackers, what's the difference? you ask... The most popular active GPS tracker will transmit its location live and allow you to log in to an online account to not only view where the tracker is currently, but where it also has been in the past.
Active GPS trackers can even send you alerts based upon speed such as if the vehicle goes above 80 miles per hour, now this is great for child drivers or commercial drivers, since its transmitting live it is also capable of alerting you if the vehicle enters or exits specified areas, this is called geofencing, it's useful to know when your kids get to or leave school.
Passive GPS trackers do very similar things to an active GPS tracker, they track location and overlay the information onto a map, the major difference here is that passive GPS trackers do not transmit any information live, meaning you are not able to view where the vehicle is currently, and you are also not able to get real-time alerts either.
Data is stored internally to the tracker itself so to view where the vehicle is or has been you have to remove it from the vehicle, and plug it into your computer, passive trackers still give you an address as well as speed so it can be very useful.
There is no monthly fee associated with this type of tracker.
The easiest way to determine which tracker is best for you is to determine whether or not you need to see where the vehicle is live.