If you repeat this on-off pattern fast enough with an LED for example, the result is as if the signal is a steady voltage between 0 and 5v controlling the brightness of the LED. To get varying analog values, you change, or modulate, that pulse width. The duration of "on time" is called the pulse width. This on-off pattern can simulate voltages in between full on (5 Volts) and off (0 Volts) by changing the portion of the time the signal spends on versus the time that the signal spends off. Digital control is used to create a square wave, a signal switched between on and off. Pulse Width Modulation, or PWM, is a technique for getting analog results with digital means. It is available in the File->Sketchbook->Examples->Analog menu of the Arduino software. The Fading example demonstrates the use of analog output (PWM) to fade an LED.
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