This is very basic but may help.
The position of the arm is set by individual axis which intern have either ac or dc motors which are linked to a closed loop system with either incremantal or absolute encoders or resolvers.
This could be explained further but takes time...
The movement of the indivial axis can be carried out in different ways, from manual motion real time i.e moving it from a teach pendant or in automatic, this is pre calculated movements using mainly algorithms inside robot software.
Velocity, ramp speeds, parameters related to the robot have to be considered for certain applications, but usually the standard settings will be ok.
So internally you get a guide a signal from the robot software which is sent to the servo amplifier units, the output from these drives the motors, the resolvers or encoders give positional feedback to the software via some electronics.
Is this brief enough.