A Visitor Oyster card is a smart card that’s pre-loaded with pay-as-you-go credit. It’s an easy way to travel on public transport in London. The card can be used on the Tube, Docklands Light Railway (DLR), Tram, London Overground, TfL Rail, River Bus, and most National Rail services in London. It’s cheaper than buying a paper single ticket, with a maximum daily cost of £7.20 for unlimited travel in zones 1-2.
You can purchase a Visitor Oyster card online before you arrive in London and have it delivered to your home. It costs £5 (plus postage) and comes with pre-loaded pay-as-you-go credit of your choice. The card does not expire and any remaining credit can be used on future visits to London, or refunded for a £5 fee.
Using the card is straightforward. Just touch it on the yellow card reader at the start and end of your journey on Tube, DLR, London Overground, TfL Rail and most National Rail services. For buses and trams, only touch in at the start of your journey. If you run out of credit, you can top up at any Tube, DLR, London Overground, TfL Rail station, and at most National Rail stations.
Go to source article: http://www.tfl.gov.uk/travel-information/visiting-london/visitor-oyster-card?intcmp=4043#on-this-page-3