ABS (anti-lock braking system) has been around for a while and is designed to stop car wheels locking whilst undergoing heavy braking, which leads to skidding and loss of control. Basically, when the car moves the wheels rotate obviously. To slow down safely, the brake pads exert pressure upon the wheels, causing them to rotate slower. This slows the car down, whilst the continuing rotation allows the car to turn under control. This is because when the front wheels turn, as they continue to rotate they pull the front of the car in the desired direction. The problem is that if one or more wheels start to rotate at significantly different speeds to the rest (when turning, the wheels need to rotate at slightly different speeds, else the car wouldn't trun), the wheels begin to lock. This can result in a wheel or wheels stoppping rotating and simply sliding along the ground. If you then turn whilst in this situatio to go around a corner, because the wheels are sliding and not turning, they cannot pull the front of the car around the corner and you will skid forwards in a straight line. Possibly the car will beign to turn but contine going in the original direction, causing you to face on way and move in another. Not good. You're off the road. So, the ABS system monitors the speed of rotation of all the wheels, and if it senses one turning at the wrong speed it can reduce or increase the brake pad pressure to allow that wheel to regain a similar speed to the rest, allowing the car to turn smoothly just like when not braking. The ABS system does this repeatedly when braking, so some drivers can feel a sort of pulse when this is active and braking. Any questions?