Fritzing Creator Kit - #09 Motor Control with an H-Bridge
An easy way to make electric motors rotate in both directions is by using an H-bridge. This is an IC* that is optimised for its use as a motor driver (L293D). It can control two motors independently.
The power is supplied by a battery, though of course a mains adaptor may also be used. If the motors are small (eg vibration motors), they can be powered directly from the Arduino. To do this, you only have to connect the orange wire of the H-Bridge to the circuit's +5 V pin.