Ok .... gonna explain this , hoping your not trying to choke my chicken here ...
A servo is a motor that is encased in a plastic housing that is digitally controlled , which when a signal is received ( from the receiver part of a radio setup ) , it acts upon the signal . Moving in the direction and amount of direction as per directed via the transmitters original signal after being received by the radio receiver .
Thus , in turn , this is what moves the device that is needed to be moved . ( throttle control on a carb , steering - rudder , or ???? The servos come in different torque ranges .... anything from ultra light weights for items like park flyers , to super heavy duties for things like battle-bots .
Generally , the ones most often found and used range from around 30 ounces of torque , up to a just over 200 ounces of torque ( the 1/4 scales are generally the ones used for high speed boats requiring lots of torque for the rudder(s) )
To view some examples , click on the following link :
http://servocity.com/html/hitec_servos.html
And then click on any of the model numbers listed and it will give you the specs and sizes of the servo.