[CivicSpoon]

LSD, Limited Slip Differential, is used to make both wheels move under acceleration. Basically it help get more traction. Like on a drag strip, instead of having the 1 wheel trying to pull your H/A at the start, both wheels are used to pull the car. Usually 1 wheel will still have more power than the other, but the opposite wheel will still be getting power to it. With autocross, when you're taking a turn, the dominant wheel may lose traction. And with an LSD, if the dominant wheel loses traction, the other wheel will still have traction and be able to power the car around the corner.
Personally I think the LSD is worth it. If you're getting a motor and want the LSD, shop around because different places chanrge anywhere from $250-400. And the lower price for the LSD and higher price for the motor at one place may make up for the lower price of the motor and higher priced LSD from another.