Getting around London by underground train.
Probably the most useful transportation method for the visitor to London is the tube, or Underground. The London Underground can be properly said to have begun in 1890 when an electrically-powered line was opened between King William Street and Stockwell. There are now 12 lines plus the Docklands Light Railway serving millions of travelers around London every day.
Although the tube will usually be the quickest and most inexpensive way of getting about London (that is, if you take advantage of the various discount passes available), be aware that the British Rail "above-ground" trains serve some areas of London better than the tube (details here).
To study a copy of the London Underground map at a larger size than the one above, click here (caution, this image from the London Transport web site is very large and takes some time to load).
Click and drag anywhere on the map to view other locationsUnderground map courtesy of Transport for London
Regular underground service runs from 0530 to just after midnight. The gap in service during the night is bridged by a night bus service.
Greater London is divided into 6 transit zones. Anytime you travel beyond one zone the fare rises. You can save money by only buying as far-ranging a Travelcard as you'll need, but do not try to use a pass to go to a zone you are not authorized for! You'll be liable for the £10 fine mentioned above, and inspectors DO check.
Take a good look at the zones on a transport map (available free at any tube station or at the London Transport web site). Most of the major tourist attractions are located in Zone 1, so you're fairly safe with a Zone 1 or a Zone 1-2 card. However, be sure to check the location of your hotel against the fare zone map. If your hotel is out in zone 6 and you will be traveling to Inner London, you'll need an all-Zone Travelcard.
Travelcards come in bewildering variety of options. These are sample prices current in 2015. For up to date prices, visit the London Transport web site.