Pulse width is the nominal control pulse width out of your receiver that will send the servo to neutral position. A standard servo these days is spec'ed with a neutral pulse width of 1520 microseconds, or 1.52 milliseconds. If you call that neutral pulse width 1500 microseconds nominal, the standard pulse width varies from 1000 microseconds with the control stick at one extreme of movement, to 2000 microseconds with the control stick at the opposite extreme.
Some servos designed for use on tail rotors with certain gyros use a neutral pulse width of 760 microseconds instead of the 1520 value. These servos, if plugged into a standard servo port will drive to one end, stall, and most likely be damaged before you figure out what went wrong.
Frame rate, measured in Hz (cycles per second) is the maximum rate at which the servo can receive new position updates. The typical frame rate out of most receivers today is 50 - 60 Hz. A new position pulse squirts at 16 to 20 times per second. Analog servos cannot handle higher frame rates due to how they process the incoming pulses. If you send them new updates too fast, the servo amplifier usually dies. Digital servos, due to their design, can accept new position updates at a much faster rate, typically 270 Hz or faster.
Since people are always striving to have screaming fast tail rotor servos, many manufacturers will market tail rotor servos having the 760 microsecond pulse width (and reduced overall travel values). The higher frame rate (270, 330 Hz) is marketed to give you "superior holding torque".
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