It used to be that you had to buy an Oyster prepaid pay-as-you-go card to get the best prices on London transport. They recently introduced contactless payment by simply tapping your credit card when you enter and exit the Tube network and have started phasing out the Oyster card. They promise to offer all of the features of Oyster including daily and weekly (Monday to Sunday) fare capping so that you always pay the lowest rate. Tube ticketing changes quite often so it’s always worth checking out the various types of passes here
Depending on how long you’re in London you might find the best option is still the Oyster card with a 7 day travelcard loaded onto it, which you can buy at any Underground station. You’re ahead by buying the 7 day pass if you’ll be using the Tube to the daily maximum for 5 days — and you’ll hit the daily maximum after about 2 trips on the Tube.
Whether you should buy a 7 day travelcard or just wave your credit card depends on which day of the week you take your first ride on the Underground. If I arrive on a Monday I wouldn’t need to buy a travelcard and would just use my contactless credit card to tap in and out since Transport for London will automatically offer daily and weekly fare capping, and their week runs Monday to Sunday. However, if I arrive on a Thursday, I am better off to buy a 7 day Oyster card that starts on a Thursday. The 7 day zone 1-2 travelcard is £32.10, and then I might add on £5 or so to my Oyster in case I travel beyond zone 2. This gives you unlimited bus and tube travel within zones 1 and 2. The daily maximum cost on Oyster or contactless credit card is £6.40 for zones 1-2. Specifics about fares can be found here.