The London Underground, or the Tube as it known locally, is the best way to get around London. There are color-coded lines which may seem confusing when you first try to find your way but with practice it can be quite straight forward. Remember the tube is not a 24 hour service.
Free pocket-sized tube maps are available at every tube station and there are poster-sized maps on every station platform and in every ticket hall.
You can view and download the official London Underground map from the Transport for London web site and isubwaymaps.com offer a free London tube map for your iPod.
My best recommendation would be to use the Online Journey Planner as it gives you a choice of routes, tell you how long the journey will be, and also takes into consideration any planned engineering works.
How much do you know about the tube? Try our: London Underground Quiz.
You may also be interested in this book by David Long: London Underground - Architecture, Design & History.