A servo combines a small electric motor and control circuit in a single compact package. This allows the servo to rotate to a specified position and hold that position. Typically servos have limited range of rotation (180° is common).
There are many servos from different manufacturers, which vary widely in price, performance and value. Fortunately, virtually all servos use the standard 3-wire connector, and accept the same kind of controlling signal (PWM signal at 50 hz). Each REV Expansion Hub provides 6 servo ports, so you can plug in a servo from any manufacturer. Also, there is a standard size for servos for FTC use, so mounts can be interchanged between manufacturers.
FTC robots typically use standard size servos, not mini or extreme size.
Commonly used servos used in FTC are the REV Smart Servo and goBILDA Dual mode Servos (25-2) and (25-3), but you should check out other servos as well. Picking the right servo for your application is a question that’s almost impossible to give a blanket answer for. To learn more, please read the Choosing a Servo section.
The most prevalent problem with servos is durability. Internal servo gears in cheaper servos strip easily when subjected to shock loads. Servos are also poor at handling lateral loads or bending of the shaft. To avoid having to frequently replace servos, choose ones with metal gears and use Servoblocks or your kit’s equivalent to prolong longevity.