^Scary: Watched this video explaining differentials and it
   actually made sense to me. Defferentials basically help you turn
   your car. Anytime all four wheels aren't perfectly straight
   (like turning), each wheel is rotating at a different speed.
   (you might think wheels all rotate at the same speed, but they
   don't). So these gear setups called Differentials ('Different'
   as in Different rotating speeds) make up for it.

   now, ever heard, 'from the wheels that slip, to the wheels that
   grip?" this video explains that too. Short video and it actually
   was quite clear, for a non-car guy like me.
   ^