I want to know more about "Servo systems" ..?? What is the role of servo systems in automation ..??

Servo Systems are usually used where closely controlled positioning or torque application is required. This is to a finer degree than can be achieved with stepper motors and will provide a smoother application of motion or torque. There are a number of different flavours of Servo System and you have to understand your load's needs and the working environment to achive the best results.
Application areas:- Rapid Pick & Place, Large Disc Storage, High Accuracy Multi-Axis Tables, Extruders, Robotic Arms and Manipulators etc.

I worked with servo motors used in Hyundai Robots. as I know, all servo systems comprise of servo motor and servo board. In higher powers, the output of servo board (PWM) drives an other board and the output of the second board (amplifier) connects to the servo motor.

in addition of Mr.Paul & Mr.Rahmat comments is should tell that in a batch process when the operation is sequential type servo system applies this system can rotate in 60 ~ 90 degree from x axis and also torque and RPM would be set via 4~20 mA.

As above precise servo systems typically use PWM output to drive an amplifier and in turn a permanent magnet servomotor. Also I designed control systems in C for the system. Bandwith and steady state error are required. There is quite a bit of information on motor models available. Usually from the vendor or check GOOGLE for books and information. The Electro-Craft Engineering Handbook by Reliance Motion Control, Inc. 612-942-3600.
DC MOTORS, SPEED CONTROLS, SERVO SYSTEMS is a book I used for years. I hope it is still in print.

Simply and by definition, the Servo System, in a Control Loop - sircuit- is that unit that anticipate the reaction of a given Controled System to an external event and transmits the anticipated calculated reaction to keep the system on a given track or to change its behavior.

For exemple, the Servo Systems in a car are, the Steering Weel Hydrolic power unit that transmits the action of the driver to the front weels when he moves the steering weel left-right.
The second servo unit, which is the most important, is the Hydrolic bracking Power unit that transmits foot presure of the driver to the brakes to stop the weels, and consequently the car.

Servo system is a master/ slave system. Need to know what control objective is (Pos, Torq, Velocity) and what resources available (Energy Power, time, space, medium, distance) to achieve, with or without feedback mechanism. Usually a tight control loop with feedback is referred to as a servo system. In process automation a large number of movements are coordinated in time and position and the sequence is very important. (Making and packaging biscuits, soaps, cheese slices, into cartons to palletising the finished cartons) There is high precision. (Not like file movement in a govt dept). Look outside the plane at the ailerons when flying at high altitude and see them flutter controlling the plane's roll and pitch you will understand servo systems.

Hi google to siemens sinamics or omron mechatrolinks you will find related topics and their related FAQ