Small radio control servos are connected through a standard three-wire connection: two wires for a DC power supply and one for control, carrying the pulses.
The parameters for this pulse are that it has a minimum pulse, a maximum pulse, 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 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 duration 
The angle is determined by the duration of a pulse that is applied to the control wire. This is a form of pulse-width modulation, however servo position is not defined by the PWM duty cycle (i.e., ON vs OFF time) but only by the duration of the pulse. The servo expects to see a pulse every 20 ms, however this can vary within a wide range that differs from servo to servo. The length of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to the 90 degree position (neutral position).
The "RC PWM" used in RC servo control acts very differently than the PWM used in any other system. (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). 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. With many RC servos, as long as the "frame 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 frame rate is irrelevant. 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).
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.
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 maximum and minimums. Generally the minimum pulse will be about 1 ms wide and the maximum pulse will be 2 ms wide.
See also 
- Ron Lund. "Servo Frequency and Center Pulse Width Information"
- Bob Blick. "Servo pulse to PWM converter" "The rate at which pulses are sent to the servo is relatively unimportant"
- Society of Robots: Servos
- Pololu. "Servo control interface in detail" "servo control signals... the frequency of the pulse train does not affect the servo position if the pulse width stays the same"
- "Introduction to Servomotor Programming"
- "Understanding PWM"
- 4QD-TEC. "Pulse Width Position Servo"
- "Servo Control"
- "Driving the Servo Motor"