In relation to the transponder technology, a squawk refers to a response that a transmitter responder (i.e. transponder) is making to an interrogation from Air Traffic Control (ATC). In a classic Mode A/C transponder, a squawk response included very basic aircraft ID, general status, and pressure altitude information.
A squitter or squit is a transmission that frequently sends aircraft ID and position data while not being interrogated. In some Mode S transponders and ADS-B devices, the squit transmission can include up to almost 50 parameters including lat/long position, direction of flight, velocity, and more.