The venerable H-bridge is a circuit for driving motors. It allows control of direction using digital I/O. It features transistors in an 'H' configuration to swap the poles of the motor on command. Many use an off the shelf H-bridge chip like the L293D from Texas Instruments. There are, however, inherent limitations in an off-the-shelf part. If you have to drive a treadmill motor, for example, these often require 90v at 10A. That's 900 watts! An L293D can't handle that kind of current. To drive one of these, you need to roll your own H-bridge. Randofo has posted this Instructable on building your own H-bridge. Link via the MakeZine blog.