A servo is a small rotary actuator, with a feedback potentiometer to verify position, and a simple feedback loop. The servo is controlled by a positive (standard +4.8 or +5V, but sometimes +6V), a ground (0V), and a control wire lead.
The control wire is fed a 50hz PWM signal. The servo is *not* run on the archetypical PWM duty cycle range of 'on' for 100% of the time to 'on' for 0% of the time, it is run on a signal established by convention in the RC/robotics world, the 'standard pulse servo mode', or sometimes 'PPM encoding'. 50hz gives a 20ms window for each pulse. Within each window, a short pulse of 5V is delivered. Usually, the pulse is between 1ms and 2ms to define the range of motion, with 1.5ms being the center of the range of motion of the servo. The servo has to receive a pulse every 20ms in order to continue to function.
- Servo Control - Wikipedia
This mode of operation allows for a relatively simple means of multiplexing and demultiplexing servo control pulses, to fit multiple independent servo positions on a single electrical channel. This is uncommonly used.
 Analog vs Digital
In both analog and digital servos, the motor is driven with a separate, conventional, digital PWM signal generator working much higher than 50hz, where 'full power' is '100% duty cycle'. They both run off the same PPM control signal.
The difference is that analog servos use a simple potentiometer voltage feedback loop, using the difference between the voltage that they get back from the pot, and the voltage they get on the control lead, to drive the motor directly for the next 18ms. The potentiometer is only sampled for position during the control pulse, at 50hz. This is an elegant, lower-parts-count solution, but it suffers from a high granularity of control.
Digital servos can sample much faster than 50hz (250hz or 333hz is typical), and they can use this to infer properties like 'angular velocity', which can be used to both refine the position much faster, and to plan a path to a position, driving the servo at full power even for small adjustments; this considerably increases the available power for things other than a 100% control range switch. The algorithms that are used here are highly proprietary trade secrets, but they are more complex than the simple proportional mode that analog servos use, and they are likely incorporate some form of a control systems concept that may be termed 'overdrive'. In general, digital servos can be looked upon as a somewhat more expensive alternative to analog servos, which are considerably more responsive and have a faster correction for small deflections ('rock-solid' is a term used to describe the higher perceived torque that results), while the practical torque and power is somewhat higher, but the maximum is technically the same.