Servo's are the little black boxes that are attached to the control surfaces on planes and heli's that moves them in response to your stick movement on the transmitter. There are many different kinds, but I'll give you a general rundown:

-The motor on the inside moves an arm on the outside of it that moves back and forth. The back and forth movement pushes and pulls on a control surface to move it up or down. Nominally, they rotate 45 degrees in each direction, but with a computer transmitter, you can adjust this. (Also, retract servos move 180 degrees in each direction to actuate mechanical retracts)
-Servos can run on either 4.8 or 6.0 volts. 6.0 volts will provide more torque and holding power than 4.8 volts, but also drains the battery a little faster.
-There are analog and digital servos. Digital servos have a small computerized unit that controls their movement, and are usually more expensive that analog servos. They do, however, provide more torque and holding strength, and this is why they would be used, usually in high performance planes and heli's.
-The different brands dont make servos that function differently, so there is no significant difference between brands. Its just that servos come in a huge variety of sizes, holding strength, torque, speed, and gear material (nylon vs metal gears for example).

-Always follow a manufacturers recommendations for servo torque for a specific plane, and plan to at least match if not exceed the recommended torque requirement. (Servos for throttle's dont have to be as powerful as those used for flight surfaces) Usually, this information is found in a plane's manual, so if your planes have manuals, you most likely will not have to come on here and ask what kind of servos to use for what application; the information will be there.

Hope this helps to inform you a little more about servos. There's still a ton more info you can and will learn about them.[sm=shades_smile.gif]