system
Given the following post, respond as if you're commenting on Reddit:
human
Why is rear locking differential better than four wheel drive?

Is it better in all situations (off road vs snow vs ice vs etc.)?
gpt
A differential allows for the driving wheels to spin at different rotations per minute therefore allowing a car to turn corners. This is because the inside wheel in a turn has less distance to cover than the oudside wheel on the vehicle in the same turn. 

Think plastic coffe cup on it's side. It will roll in a circle / turn because the bottom is narrower than the dinking end of the cup. What a differential essentially does is give power to the wheel with the least resistance. This is a desirable trait in wheels on a road surface, because otherwise the driving wheels start to squirm and bounce. 

In mud however this is not, because when a drive wheel looses traction and starts to spin the power of the engine will just be sent to the spinning wheel and therefore not adding to the traction of the vehicle. To overcome this some cars can lock their differentials to ensure that both wheels will start churning at the same speed giving a lot more traction to the vehicle.

On a side note: don't lock your diffs on a road surface (if your car has the option). That is, unless you really like your mechanic, and like to give him, or her, fist fulls of money.