A limited slip differential is a kind of differential gear used in automobiles.
Cars need a differential gear. They make up for the fact that as a car turns, the wheels on the left and right must rotate at different speeds. The front wheels on a rear wheel drive car is no problem, as the wheels can simply not be attached to each other. But the rear wheels on a rear wheel drive car (or the front wheels in a front wheel drive car) must be attached to the drive train, and therefore attached to each other. A differential gear allows these connected wheels to rotate at different speeds, while still being attached to the drivetrain (the engine and such).
The problem with a conventional, or 'open' differential is that when one of the wheels slips, the drivetrain is not able to provide power to the other wheel. Basically, if one of the wheels starts to spin (not gripping the road), and additional power travels through the past of least resistance - the spinning wheel. Though this is usually not a problem, cars designed to go fast (race cars and sports cars) could go faster if this didn't happen.
Thus, the limited slip differential. The construction is such that even if one of the connected wheels starts to slip, some power from the drive train is applied to the non-slipping wheel. This allows more power to be translated into speed in these conditions. Cars with limited slip differentials (or LSD) tend to accelerate faster, but are harder to turn, than non-equipped cars.