http://resources.infosecinstitute.com/man-in-the-middle-demystified/
You can also view a how-to video tutorial that you can follow along with if you have a few virtual machines to play with on your local network:
http://resources.infosecinstitute.com/video-man-in-the-middle-howto/
In a pen test, it is important to learn how to do these attacks to intercept server to server communication, server to client communication, etc.
Coming soon we will demonstrate how to perform a MitM attack against SSL encrypted sessions.
Red more: InfoSec