A tracker is a type of APRS station. The intent of a tracker is to transmit position reports. To do this a transmitter, TNC and GPS are required. Technically you don't have to have the GPS if there is some other way to obtain position information (like typing it in or selecting it from a map display), but having a GPS makes things automatic.
Trackers can be attached to a vehicle, a person, a balloon, or anything that moves around.
Using a map display along with a receiver (or the APRS-IS internet system) the accumulation of position reports appears as a track on a map.
Every APRS transmitting station, trackers included, has to have a unique identification callsign-SSID pair. The tracker should normally use the identification callsign of the licensed radio operator acting as the control operator.
In special cases a tactical callsign may be used as the name of the station instead, as long as normal station identification requirements are met. Usually this involves adding the station callsign into the comment field or a separate status packet periodically, but there are other methods that satisfy FCC rules.
Notice that no receiver is required. This means that many trackers blindly transmit, and cannot serve as digipeaters, I-Gates or other more sophisticated roles. Blind transmitting means that the tracker needs to be configured in such a way as to avoid unnecessary interference. So trackers are usually low power, 5-watts or less output, and the interval of position reports is the largest interval compatible with the goals of the tracker operator.
Stations with receivers can perform a tracking function along with other roles.
Trackers can be made to various degrees of sophistication. Some of them are smart enough to change the position report sending rate based on speed or directional changes. Trackers usually don't have much of a user interface. It is sometimes possible to program the tracker with a computer connection before it is put into service, then the only control might be an on/off switch.
 Legal Implications?
A tracker is a radio transmitter so it has to be controlled by a licensed radio operator following the laws and regulations which apply in that jurisdiction.
In the USA, it is generally acceptable to have the tracker be controlled by a licensed operator, who could turn it off if it were to start causing interference. (Don't take this as legal advice!) The idea of control is not strictly defined. It may be that the tracker is not physically within grasp of the controlling operator, but that he or she can access it within a reasonable amount of time if intervention is required ("reasonable time" not being carefully defined either!). Usually trackers are not configured for remote control via DTMF, telephone, or other methods as employed in the voice repeater realm.