So I talked before about servos and what they are, but I decided I like spinning things anyway :)

So I suppose I will start with the RC servo, maybe take a look and see what the innards of the Parallax one actually does.  So per this site, RC servos have:

DC electric motor Gears with an output shaft Positioning sensing mechanism Control circuitry

The controlling intelligence, in this case the operator of the model, indicates to the servo the position that the output shaft should have. The position-sensing mechanism tells the servo what position the shaft currently has. The control circuitry notes the difference between the desired position and the current position, and uses the motor to "make it so". If the difference in position is large, the motor moves rapidly to the correct position; if the difference is small, the adjustment is more subtle. As for the operator, all he knows is that he moved a slider half-way up, and the rudder on his model plane moved to the center position, and will stay there until he moves the slider again.

This is a lot like what JohnS_AZ had mentioned previously.

Inside the box is an electric motor which rotates the arm that sticks out the top.  There is ALSO a small potentiometer that measures exactly where the arm is so when you hook it up, it can give that information to the MCU.

When you send a PWM signal to it, that signal represents where you want the arm to point.  So a little circuit inside the servo compares your PWM signal to the measurement from the internal pot, and ends up with an error term.  It then drives the motor one direction or the other to make the internal pot match your PWM signal.

And the article goes through different connections that different manufacturers use (in terms of color) and style, and for the most part, a colored line is the PWM signal, the white is power, and the black is ground. 

It seems that when you do pulse width modulation, the width of the pulses sent control the position to take.  So

pulse width          angle                     comment 0.6m Sec         -45 degrees       minimum pulse length 1.5m Sec           0 degrees             center position 2.4 mSec         -45 degrees       maximum pulse length

Another example - you notice the time for one pulse (on and off) is equal to other pulses, but the actual on portion of the pulse changes and that determines direction of the servo.