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Servo control is achieved by sending a servo a PWM (pulse width modulation) signal, a series of repeating pulses of variable width where either the width of the pulse (most common modern hobby servos) or the duty cycle of a pulse train (less common today) determines the position to be achieved by the servo. The PWM signal might come from a radio control receiver to the servo or from common micro-controllers such as the Arduino.
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Small hobby servos (often called radio control, or RC Servos) are connected through a standard three-wire connection: two wires for a DC power supply and one for control, carrying the control pulses.
The parameters for the pulses are a minimum pulse width, a maximum pulse width, and a repetition rate. Given the rotation constraints of the servo, neutral is defined to be the position where the servo has exactly the same amount of potential mechanical rotation in the clockwise direction as it does in the counter clockwise direction. It is important to note that different servos will have different constraints on their rotation but they all have a neutral position, and that position is always around 1.5 milliseconds (ms) pulse width.
Pulse duration
In modern RC Servos the angle of mechanical rotation is determined by the width of an electrical pulse that is applied to the control wire. This is a form of pulse-width modulation. The typical RC Servo expects to see a pulse every 20 ms, however this can vary within a wide range that differs from servo to servo. The width of the pulse will determine how far the motor turns. For example, in many RC Servos a 1.5 ms pulse will make the motor turn to the 90 degree position (neutral position). The low time (and the total period) can vary over a wide range, and vary from one pulse to the next, without any effect on the position of the servo motor.
Modern RC Servo position is not defined by the PWM duty cycle (i.e., ON vs OFF time) but only by the width of the pulse. (This is very different from the PWM used in some other systems. In particular, the confusingly similar-sounding direct PWM DC motor speed control works entirely differently). Most RC Servos move to exactly the same position when they receive a 1.5 ms pulse every 6 ms (a duty cycle of 25%) as when they receive a 1.5 ms pulse every 25 ms (a duty cycle of 6%) -- in both cases, they turn to the center position (neutral position). With many RC servos, as long as the refresh rate (how many times per second the pulse is sent, aka the pulse repetition rate) is in a range of 40 Hz to 200 Hz, the exact value of the refresh rate is irrelevant.
The period of 20 ms (50 Hz) comes from the days where the signal was encoded in PPM (Pulse Position Modulation) format to be sent over the air. The PPM period was around 22.5 ms, and the conversion to PWM was trivial: the time of the PWM high state was the time position of the PPM Pulse for that servo.
Most RC receivers send pulses to the RC servo at some constant frame rate, changing only the high time. However, it is possible to command an RC servo to move over its entire range with a function generator set to a constant 10% duty cycle by changing only the frequency (frame rate).
Force
When these servos are commanded to move they will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo will resist from moving out of that position. The maximum amount of force the servo can exert is the torque rating of the servo. Servos will not hold their position forever though; the position pulse must be repeated to instruct the servo to stay in position.
Variations
When a pulse is sent to a servo that is less than 1.5 ms the servo rotates to a position and holds its output shaft some number of degrees counterclockwise from the neutral point. When the pulse is wider than 1.5 ms the opposite occurs. The minimal width and the maximum width of pulse that will command the servo to turn to a valid position are functions of each servo. Different brands, and even different servos of the same brand, will have different maxima and minima. Generally the minimum pulse will be about 1 ms wide and the maximum pulse will be 2 ms wide.