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AnalogOutput

Analog output with Arduino is based on PWM which stands for "Pulse Width Modulation". PWM is the practice of pulsing an output pin up and down in a very rapid sequence. While the Arduino cannot output voltage other than 0V or 5V, by writing a pin rapidly high and then low, we can output an average voltage over a very short time (a few milliseconds) that mimics a voltage between 0V and 5V.

To write out voltage, we specify a number from 0 - 255 and write that number to one of our digital pins labeled with the "~" symbol.

Here is a link to the Arduino page on PWM.
http://arduino.cc/en/Tutorial/PWM

PWM Page on Wikipedia - probably a lot more info than you need though :)
http://en.wikipedia.org/wiki/Pulse-width_modulation

PWM is necessary for doing things like:

• controlling servo motors
• fading LEDs
• mixing colors for RGB LEDs
• changing motor speeds
• any output that requires more than a simple "on" or "off"