Oyster
Oyster is Transport for London's pay-as-you-go smartcard for the Tube, buses, trams, London Overground and most rail services in the capital. You touch it on a yellow card reader when you start a journey and, on rail and Tube, again when you finish, and the fare is deducted from its balance.
In practice
Today most people simply use a contactless bank card or phone, which charges the same pay-as-you-go fares as Oyster. Both are capped: once you have spent a set amount in a day, or in a Monday-to-Sunday week, further journeys in the zones you used are free. Oyster still matters if you have a railcard discount loaded onto it, if you want to keep travel off your bank statement, or for visitors whose foreign cards charge fees. Always touch in and out with the same card or device, or capping will not work.
Why it matters to newcomers
You do not need to buy a paper ticket for travel within London. Register your card or device with TfL to see your journeys and claim refunds for incomplete ones. Buses do not take cash, so carry a card or phone that works.
Related
- Zones - how fares are set
- Night Tube - the same cards work through the night
- ULEZ - TfL's other charge, for drivers
Source: Transport for London: how to pay.