To supply the power to the drill.

Mainly that one is a drill, and the other, a screwdriver.

SOME Power Drills are made that can do both but not all power drills will do this - you need to read the instructions for the drill.

Power is transmitted between the drill spindle and drill shank through friction

drive (Taper shank push fitted into spindle taper)

The only difference between a battery drill and a regular drill is the type of power source. A battery drill is one that does not require a power cord, but can only last a finite amount of time.