Visitor Oyster cards are plastic smartcards you can use instead of paper tickets. Put on pay as you go credit which you use when you travel. It is the cheapest way to pay for single journeys on bus, Tube, tram, DLR, London Overground, Tfl Rail and most National Rail services in London.
From January 2016 you can also use your Visitor Oyster card to pay for travel on train services to/from Gatwick Airport, including Gatwick Express