This circuit will simulate all the valid states of an H-Bridge, but without using actual transistors. This is ideal for somebody that wants to learn how it works, and want to truly understand what happens.
Study the circuits above well, or even better than that, build it on a breadboard, and play with it. That will give you a great understanding of what it does and how it works.

The next stage of our journey, is to start using transistors to build the circuit above, but before we do that, we first need to see exactly what a transistor does when used as a switch, and what this NPN and PNP stuff means, and also how it affects our circuit ...

I will not go into transistor theory in this article, but the diagrams below will give you a clear idea of what transistors can do and how they do it as well.

Now that we had a look at these circuits, and hopefully built them on a breadboard, lets construct half of our bridge, so that we can start to see how to change the circuit with switches only, into a circuit that you can control with a micro controller like Arduino.

Well done, hopefully you have built this one on the breadboard as well, and may be anxious to see the full circuit.
Well, lets not keep you waiting then.