Most airplanes are fitted with a device called an ATC Transponder, which detects the radar beam from an airport or enroute surveillance radar station. When the transponder picks up the beam, it sends out a 4-digit code number in the form of a radio signal carrying the code number as a digital number. Most transponders are also connected to an "encoding altimeter," and can send the airplane's altitude as well as the 4-digit code.

Sending out this code is called "squawking," so the code is called a "squawk code."

The pilot of the airplane can enter the 4-digit code through the dials on the front of the transponder, which is mounted in the airplane's instrument panel. Whenever you contact Air Traffic Control (ATC) you will be assigned a squawk code by the controller.

There are certain standard codes; for example, the code 1200 means "I am flying visually on my own and not calling on ATC services." Pilots call this "squawking a dozen."

There are other codes that mean, "My radios aren't working" or "I have been hijacked." Aside from those few, there really isn't a list. A squawk code can be any four digits in the range 0-7.

A computer connected to the ATC controller's radar console picks up the codes and displays associated information on the radar screen so that it follows the airplane around on the screen. This gives the controller much clearer information than just the "blip" on the screen that they had with the older radar equipment.

For much more information including pictures, tables, and diagrams, try an internet search on "ATC transponder" or "squawk codes."