ABS - Anti-lock braking system or anti-skid braking system is a safety system in automobiles, 2 wheeler and also in airplanes. As the name itself reveals, ABS keeps the wheels of the vehicles from locking up and while applying brakes and helps the drivers to handle their steering wheel.
The skidding wheel has less grip on the road than the non skidding wheel. If your car drives in a watery road or ice covered road, the traction of the wheel on the road will be less. Which means the wheel will keep on rolling, but the car will not move. If in a highway, your car is more than 120 mph speed, and you want to apply brake to avoid accident, it is because of ABS only you can able to do it.
During those times ABS changes the brake fluid flow and pressure, irrelevant to the pressure of the brakes applied and bring the pressure of the break to the required level.
ABS has 4 components :-
1.speed sensor - It monitors and controls the acceleration and deceleration of the wheel . It consists of exciter and coil/magnet assembly .
2.Valves - Valves regulate the air pressure during ABS action. There is a valve in brake line of each brake while the ABS is in action.
3. ECU - Electronic control unit receives the signals from the sensors in the circuit and controls the brake pressure.
4.Hydraulic control unit - The hydraulic control unit receives signals from electronic control unit and apply or release the brake according to the need.
Like this ABS is preventing wheels of the vehicles from locking when in skiddy roads.
Its a simple phenomenon to avoid skidding. When the driver is driving faster and suddenly if he faces any obstacle (like any *****, a vehicle on wrong side of road, any one crossing road etc.,) he has to brake harder. When you brake hard suddenly for a long time the chances of skidding is high(especially on suface covered with sand or porous mud)
So an electronic embedded system is designed so as to lock brake and leave the brake in the form of pulses. But this locking and leaving occurs at a time frame of micro or nano seconds. This helps the driver to maintain balance in two wheelers and steering control in four wheelers.
This was especially designed for disc braking system as the callipers have a harder grip over disc there by sudden braking.
There are many ABS demonstration videos available on youtube. Can can check them out to know the proper functioning. Hope this helps you out.