limited slip, positraction and trac lock are basically all the same thing. They are units that go in the differential and keep both the tires turning the same speed but slip around corners. All of these contain frition plates and steel plates and springs that hold them together. Pretty much like how an automatic transmission is put together. Posi is chevys name for their unit, traction lok is fords name for their unit, they can be rebuilt. I think limited slip is just the generic term.
A spool is pretty much like a solid peice of metal holding the ring gear and the axles together. You can make a home-made spool by simply welding the spider gears together. These are not streetable though and will chirp around every corner big or small. Good for racing.
Then there is a detroit locker, which is also good for racing but clicks and chatters around corners. There is an Auburn locker but i dont know much about them except they are rebuildable. There are air lockers also, which work as an open diff. all the time but when you flip a switch, it locks up like a spool. They also have lockers that work the same way only with elecricity. The best kind of differential....thingy would be the torsen torque sensing differential. They put these on the cobra R, BMWs have them and many other of the most prestigious cars world wide. They litterally sense which wheel has the most traction and gives more power that wheel.