Usually near an airport, you will see a spinning RADAR sensor. This is actually a transmitter and 2 receivers. There is the pinger that sends out a signal, and a receiver that will hear the reflections off of aircraft, buildings and clouds. There is also a second receiver that will receive the results replied from a transponder equipped aircraft.
The pinger receiver will allow the radar display to know the range and azimuth of the object sensed, as well as some degree of intensity. The intensity is used to distinguish aircraft from weather, and buildings.
The transponder receiver will offer an azimuth, a guessed range, and some data offered by the aircraft. The data will usually have a beacon code (mode A), altitude (mode C), and potentially other information (mode S).
In the US there are usually two types of RADAR Sensors; a long range, and a short range. The long range sensors will usually "see" about 250 miles around them (depending on terrain). The short range sensors will usually "see" about 60 miles around the sensor.
The Air traffic control people can use this sensor data to control aircraft in their area.
wikipedia.org dumped 2003-03-17 with terodump