The system is fairly straightforward. You can have paper tickets, an Oyster card or a contactless card/device. The Oyster card and contactless offer the benefit of cheaper fares (single use paper tickets are more expensive than paying with an Oyster or contactless card) and flexibility (On days you don't travel as much, you only pay the individual journeys. On days where you travel a lot, the daily/weekly caping will make sure that you never pay more than £ X (X depending on what zones you travel from/to/through).