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There are two basic options for paying fares – either a Visitor Oyster (smart card) or a paper Travelcard.
The Visitor Oyster Card scheme makes travel on public transport much easier and faster, and can be bought and delivered to you before you arrive in the UK. It is valid in all travel zones and automatically calculates the cheapest total fare for all the journeys you make in a single day.
The paper Travelcard is a pass which is valid for either a single day or seven days, and is available for certain combinations of travel zones. It is also valid on all public transport on the TfL network.
Whether you should get the Travelcard or Oyster card depends on how long you’re staying in London and how frequently you’ll be using public transport.
If you are in London for a week and plan to use only the central London Travelcard zones on a daily basis then a 7 day Travelcard might be the most cost-effective option. If you plan to stay in London for a shorter time or want to travel to the outskirts of the city, the pay-as-you-go Oyster card cost might be cheaper.
For more information to help you decide which is best for you:
https://www.visitbritainshop.com/world/london-transport-which-ticket-to-buy/
https://www.visitbritainshop.com/world/help/travelcard-or-oyster-card/