Arduino/DC Motors

Driving them.

On and Off (Transistor)

This is called bang-bang control. You either have full voltage or 0 voltage.

Since the Arduino can only supply a few hundred mA it is best to power them from an outside source.

Pulse Width Modulation (PWM (Transistor))

This feature is built into thje arduino The pins are labeled with a wavy symbol

On the UNO 3, 5, 6, 9, 10, and 11 have PWM capability.

All you are doing is switching the power on and off to simulate a lower voltage.

PWM through transistor