What is an Oyster card?
Most of London's public transport tickets are contactless, and many locals have started using their contactless debit cards, and Apple/Android Pay instead of Oyster. Oyster card is still a good option as it allows for managing the amount of money you deposit on the card and for people who don't have a personal contactless card. You should also check with your bank or try the debit card in case there are any problems with it, especially when travelling from abroad. Paper tickets are still in use on commuter trains, and the underground but an Oyster Card/another form of contactless payment is significantly more convenient. You cannot buy tickets on buses as these need to be purchased in advance.
Why should you get one?
The daily maximum for an Oyster card or another form of contactless payment depends on the zones you want to travel in. In Central London (zones 1-2), the daily cap is £7. If you need to go further out, the daily cap for zones 1-4 is £13.10. Along with saving money, the card is easy to use, and you can make sure that you don't spend more money than you deposit. You can use it to travel via underground, bus, trains and even the Thames Clipper by using the card at the respective card readers. You should use the same card throughout the day so that you can make the most out of the maximum daily fare. This is the case even when the money is going out from the same bank account, for example, when using a debit card and Apple Pay.
At underground stations, the card readers are at the entrance gates. On buses, the card reader is next to the driver as you enter the bus. Many buses also have a card reader in the back. Many train stations have gates where you need to tap your card although in some cases there are no gates and you'll need to find a card reader and use the Oyster/contactless there. In the evenings, some of the stations further out in the city have their gates open, but you should still tap in/out with the card. The staff on the Thames Clippers will go around and ask for tickets, and you can use your Oyster to pay.
Where can you get one?
You can buy an Oyster card from most underground stations as well as kiosks around the city. It can also be used to travel from the airport (Heathrow, Gatwick) to central London, so you can get an Oyster when you arrive.