Electronic Stability Program
More commonly known by the generic term Electronic stability control (ESC).
ESC is a computerized technology that improves the safety of a vehicle's handling by detecting and preventing skids. When ESC detects loss of steering control, ESC automatically applies individual brakes to help "steer" the vehicle where the driver wants to go. Braking is automatically applied to individual wheels, such as the inner rear wheel to counter oversteer, or the outer front wheel to counter understeer. Some ESC systems also reduce engine power until control is regained.[1]
Starting with MY2001, all S4s come standard with ESP. A button inside the cabin "disables" ESP's throttle intervention, but not brake intervention. In order to fully disable ESP, fuse #42 must be pulled from the main Fuse panel between the driver side door and the side of the console. This disables power to the ESP hydraulic pump.
ABS
Antilock Brake System
Selectively pulses braking on wheels that are showing signs of locking up
EBD
Electronic Brake-force Distribution
Selectively applies braking to wheels if the ESP controller detects "unsafe" yaw
Can be disabled by unplugging fuse 42
ASR
Anti Slip Regulation (a.k.a. traction control)
Reduces requested torque if the ESP controller detects wheel spin during acceleration, especially while turning
Can be overridden in the ECU
EDL
Electronic Differential Locking
Applies braking to a wheel that the ESP control detects is losing traction, thus redirecting torque to the opposite wheel (which presumably has more traction) through the open differential.
Can be disabled by removing fuse 42 or hardwiring the emergency brake signal input to the ESP/ABS controller to +12V (switched)
MSR
Motor Slip Regulation
Increases requested torque if the ESP controller detects wheel slip due to a sudden decrease in engine speed (torque braking), such as in sudden off-throttle conditions.