Oyster VS Travelcard
This is something I wish I had known the few times I visited London before moving to the UK: it did take me a while to understand it. At that time, this campaign didn't exist (I only started seeing these useful signs in the underground a few months ago). I'll even go further than what the sign is saying by advising people to buy an oyster card instead of a travelcard when they visit London, even just for a couple of days. And this advice is even more valid if you're staying in Central London and might prefer walking or taking the bus instead of the tube. The Oyster Card either works as a season ticket (weekly/monthly/yearly) or on a pay as you go basis. With the pay as you go system, you top up the amount that you want and the journey price is deducted at the end of each travel (just "remember to touch in and out", another campaign everywhere in the tube). The reason the oyster is as good if not better than the travel card, is the capping system: