The Oyster Card is used widely on the London Underground (Tube) and Buses and now on the London Overground (a few rail lines). Overseas visitors may acquire a pay-as-you-go Oyster Card with an initial balance of £10 or £15 pounds. These cards are swiped through a machine at a point of entry to the Underground and once again upon exit. The fare is charged to the card and the balance appropriately reduced. On buses you need swipe a card only once upon entry. When the balance is running low, you may top them up at station machines using your credit card or cash.
For further details see the TfL Site.
A Travelcard (daily, 3-day, 7-day and Monthly) is the next best option after an Oyster card. These may be bought at Tube stations.