The Oyster card is a form of electronic ticketing used on public transport in Greater London in the United Kingdom. It is promoted by Transport for London and is valid on travel modes across London including London Underground, London Buses, the Docklands Light Railway (DLR), London Overground, trams, some river boat services, and most National Rail services within the London fare zones.
More information on the Wikipedia page [1]. The TfL page is [2].