OK lets get a lil in depth all fwd transmissions have a differential or commonly reffered to as "diffs". There are three types as 96coupe has stated. The first is an "open", in theory this diff is good for everyday use. It allows one wheel to spin at a different rate then the other. Manufacturers put these in to keep there cars ride nice and mudane for the everyday commuter. Picture this, your taking a turn (say a right turn just for example) now your inside wheel (which will be the right of course) has to travel a shorter distance then, the outside whell (being the left) if you were to lock these the two drive wheels together you would get an awful kind of bump, screeching, draging effect during turns because of the speed differential. This is why most cars come with open type diffs from the factory. Also commonly reffered to in the street racing world as "peg legged".
The second being a "LSD" which stands for Limited Slip Differential. There are many kinds of these ranging form "viscous coupling", "clutch pack", and "Helical Gear" types. The purpose of an LSD is to lock both drive wheels together under high loads. Thus enabling more traction while accelerating. This is why most higher performance cars come with them from the factory ie. ITR (integra Type R). But wait, thats not all, being a limited slip it still offers the comfort of everyday driving by allowing the speed differential between the tires through a turn, hence the name. The main advatages to these is better lauches, better acceleration out of turns, and the streetability of them. There are many aftermarket companies out there that make them a few being KAAZ, Quaife, and Phantom Grip. Phantom Grip actually sells a conversion for your stock LSD's, some people refer to them as " Phantom Slip".
The thrid is a "Full Locker" these aren't very good for the street at all. Mainly for muscle cars but you will hear of people welding there spyder gears or diff together. Ultimately this permanently locks your drive wheels together allowing for ultimate staight line traction. If you see a car going around a turn slow and its doin' the bump, screech, tire drag beware there is probably a lot of power under there hood and a truck and trailer not to far away. HOpe this shed some light on the subject for ya Keep it real
