You can try google for more in depth definition. VLSD stands for Viscous Limited Slip Differential.
The stock viscous limited slip works by immersing a series of round plates, alternately attached to an axle or the differential housing, in a fancy viscous goo. When you go around a corner, the outer wheel will turn faster than the inner wheel, but because the differential housing itself is going at a speed halfway between the two wheels, the differential housing sees the outer wheel turning forward and the inner wheel turning backward.
The plates can be stacked various ways, but let's assume that on each side of the differential there are eight plates, with half of them attached to the axle, and the other half attached to the differential. Call the axle plates "A" and the differential plates "D", and they will stack up like DADADADA.
The plates in a viscous limited slip don't actually touch each other. Instead, they are separated by a thin film of this viscous goo. When you go around a corner, and the A and D plates start turning at different speeds, the goo trapped between them heats up. Some chemist somewhere figured out how to make a magic fluid that expands and thickens under this shear load, making it more difficult for the plates to go different speeds.
While the fluid tries to lock the plates together, the tires try to rip them apart. At low speeds and gentle turns, the fluid doesn't get thickened much and the tires win. Boot it and try to spin one tire and the plates suddenly are going dramatically different speeds. The fluid thickens just as dramatically and, with the stock tires, the fluid wins, locking both rear tires together and letting you have your glory slide. A little less throttle and you have the smooth, seamless acceleration Nissan engineers had in mind. If you have really sticky tires, though, the tires win, and the long, gooey molecules of the stock diff's viscous snot get sheared into smaller, less thermo-reactive visco-thickening stuff; stuff that would make that mysterious chemist blush with embarrassment.
