LSD job is to limit the difference in wheel speed from the inside wheel versus the outside wheel. You see when turning around a corner the outside wheel has to travel a further distance, thus it has to turn MORE times around then the inside wheel. The results is two wheels (right and left) have different spinning speeds is they have different grip levels, this causing loss of traction so the wheels slip and you lose control. The LSD can lock the inside wheel and the outside wheel together via the driveshaft, this forces each wheel to rotate at the same speed. So the 3 basic settings work like this:
Inital - this effects how quickly the LSD reacts to the tires spinning, it also an overall power setting for the whole LSD system, turning it all the way down means the Accel and Decel settings don't really effect anything.
Accel - this effects how "locked" the wheels are when accelerating. On an FR (front engine / rear drive) configuration turning this up creates a "posi" style rear end, making it harder for the wheels to spin at different speeds. Think of it as a percentage, the higher number the more the wheels will be held together at the same speed.
Decel (braking) - opposite of the Accel setting, it keeps the wheels locked to each other as the car is slowing down.
If you turn up the LSD too much the car will understeer as the wheels attempt to run at the same speed which forces the car to only going straight ahead. However if the car is spinning out due to oversteer (too much wheel slip) turning up the LSD should fix it. Even FF (front drive / front engine) car can benefit from LSD as they spin their tires and lose grip when accelerating out of a turn.