They way we define any servo is through a ratings system. This allows us to compare the properties of a servo and help us determine which servo is suitable for the need at hand. There are three main ratings to be aware of on a servo.
The ratings are:
A. Torque
B. Speed
C. Voltage
What Do All The Numbers Mean? Let's break it down and explain each one.

Torque:the unit is KG · CM, Can be understood when the object’s horizontal distance to the center of the rudder shaft in the tiller is 1CM,how heavy weight the servo can drive .
Speed:under no load condition turned over 60 degree's angle in required time ,is to measure the velocity of servo.
Voltage:sometimes it can improve the motor speed and torque. Voltage selection also need to look at our controllors can provide.