Transponder

A transponder (transmitter-responder) is an electronic device installed in an aircraft that receives radar interrogation signals from ground-based Secondary Surveillance Radar (SSR) and automatically transmits a coded reply. This reply allows ATC to identify the aircraft on radar and determine its altitude.

Transponder modes:

Mode A: Transmits a four-digit identification code (squawk code) assigned by ATC
Mode C: Transmits Mode A identification plus pressure altitude
Mode S: Advanced mode transmitting unique aircraft identification, altitude in 25-foot increments, and additional data. Required in most controlled airspace worldwide

Standard squawk codes include:

7700: Emergency
7600: Radio failure (communication loss)
7500: Hijack / unlawful interference
1200: VFR in the United States
2000: Default code when entering a new FIR without a specific assignment