Limited Slip Differentials (LSD)
Open Differential: one wheel wonder, good for grandmas and groceries
Viscus Limited Slip Differential(VLSD): think torque converter, there is no "lock up" here, its friction or heat sensitive and will "push" the wheel facing greater resistance close to the torque out put at the wheel facing smaller resistance but will not match(lock up) as there is no physical or mechanical direct link. good for DD.
Helical Limited Slip Differential: S15, does "lock up" if both wheels stay in contact with the ground. I'm sure its too expensive to bother.
Clutch Type Limited Slip Differential: three types, severity of lock up is based on number of clutch plates.
1-way lock up under acceleration, open differential behavior under engine braking, for rotation in tight corners.
1.5-way lock up under acceleration, with less than lock up under engine braking
2-way lock up under acceleration, lock up under engine braking
"Welded Diff" same idea as a drag racing spool, both wheels turn together always, very hard on the drive line, very abusive to tires as one wheel is always breaking traction in any amount of turning. As far as drifting is concerned this will always be the most consistent rear end you can use. buy stronger wheel studs.
My recomendations:
If you daily drive you drift car, save up for a 2-way, if you don't weld it.
And don't for get to change fluid in all of these, or kiss it good bye.