What are Antenna Trackers ?
Antenna Trackers are hardware devices that receive a telemetry data stream from your on-board your plane. It tells your tracker where in the sky you plane is. Some of them use the GPS information from your OSD on altitude, GPS location etc and send the data down the audio channel of your VTX o your ground station.
You can get units that don’t require a OSD and transmit the data for tracking.
The tracker hardware on your ground station listens for the data on the audio channel, receives it and works out which way the plane is and how high from your location. Knowing its own position it can then send a signal to two servos. The two servos are in part of your tracker rig.
The rig can move your antennas on 2 axis, left to right and up and down, or pan and tilt. The signals tell one servo how much to the right of left to pint the antennas ( Pan ) The other signal moves the antennas up and down ( tilt )
The reason for the tracker to to point high gain antennas at your plane at all time so the antennas can pick the signal up. As the plane moves around the sky the antenna tracker follows it.
Very cool stuff 
So what systems are we looking at ?
Quiet a few manufacturers make the tracker senders and receivers. They can be bought in many forms. One such example is the :
EZTracker :
This is an example of the tracker unit. This unit is part of your ground station. The unit is supplied by a 6 volt supply. It can be powered of a Bec. It has two servo outputs, one for panning the antenna and the other for tilting it. It has USB connectivity so you can upgrade the firmware. and works off a audio channel to receive its data. Priced around £70 its a extremely accurate and practical solution. To make this a complete tracking system though you have to have either a :
EZOSD :
The EZOSD is an example of a OSD with full telemetry data output. It has on board GPS which it receives the positioning data from.It sends the data down an audio channel to the tracker. The data si received and decoded by the tracker which then calculates any movement for the servos to point the antenna directly at it.
Priced at around £150
Or a Tiny Telemetry :
Priced around £110. This is the option if you don’t want a visual display, want to keep weight to a minimum, or use a separate OSD altogether. The Tiny doesn`t produce a picture, it just sends the data to the tracker. Its equipped with its own GPS sensor so doesn’t need any other equipment other than a power supply to work.
Other systems from companies like Eagle Tree are available.
Once you have your OSD or telemetry hardware to send the data, a tracker that receives it 
you need a antenna tracker “rig”. Companies such as servocity do the hardware that you can attach your servos to that you can then mount your antennas too. Which type you choose and how you mount them is a personal choice and the sky is the limits.
For a beginner system an antenna tracker system is not advised, its overkill and not really needed. But as us boys like toys certainly worth considering as you progress