The servo is the muscle of the control surface. They act and react according to the signals received from the receiver. When purchasing or deciding to purchase servos there are a number of things you need to know.
There are may manufactures of servos. Each brand designed to meet specifications that should be known prior to ordering. You must also know your plane and the servo required, not only to fit in the servo bay, but also to provide the appropriate amount of travel and torque to operate the control surface correctly. Due you research to ensure you buy the correct servo for your application.
For example, a standard ball bearing servo may work fine on a 40 size trainer plane, but would not be of sufficient specs. to operate the ailerons on a 100 CC aerobatic plane.
Contact the servo manufacture you intend to use and discuss the best possible fit for your application.
Visit the Servo Data Base for a complete selection chart of servos.