hi..
it depends on the design or how you want the robot serve you..maybe you just need to drill a hole using x and y axis...a bit complex if you have the z axis..

robot arms may have a hydraulic lines to fire hydraulic valves that produces the action..say grip on a metal plate..you have sensors that tells you the position of the arm..mostly inductive/capacitive sensors and sometimes photo electric...you may need the services of an encoder too to give you the feedback position of the workpiece before you do any action..

robotics is a very broad topic...it is a combimation of electrical, mechanical, computers and a COMMON SENSE. strong used of imagination is necessary for you to visualize how the action will take place.

dydt

Broad question. Not much info around. Try.
http://www.roboknowledge.com
I suggest you ask there , they tend to answer even if the answer is not on the site.
I think the site is new.

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.