Three way switches are used anytime when lights are controlled by two different switches. For example the switches at the top and bottom of the stairway are both three ways.
There are many video tutorials on how to wire three way switches. This tutorial was designed to show how the switches works instead of wiring them. This should lead to a better understanding on working with the switches and troubleshooting them.
This is the first of a series of tutorials on switches. If you like this video then you may want to take a look at my Four-Way Switches & How They Work