reffer to the following links:
what is servo motor? how does it works?
http://www.engineersgarage.com/articles/servo-motor
servo motor interfacing with microcontroller..
http://www.engineersgarage.com/microcontroller/8051projects/servo-motor-project-circuit
http://www.engineersgarage.com/embedded/avr-microcontroller-projects/atmega16-servo-motor-circuit