"***Jos***" wrote:

For example: what is an Oyster card??

An Oyster card is a prepaid, stored value, RFID card. The idea is that you pay
in advance for what you think will be your transportation costs for a period of
time and the value of your payment is stored on your card. As you enter and exit
either the Underground or a Bus, you wave your card over a reader. As you exit
the system, the appropriate fare is deducted from your card.

You can buy them anonymously, or you can register your name and address. The
advantage to registering is that any value on the card when lost can be
transfered to a replacement card.