In my mind, it's one of the easiest ways to get around London's amazing network of transportation options.
It's a small, credit card sized card you can use to tap in or tap out of public transport such as the London Underground.
Topping up an Oyster Card is quick and simple!
It's a £5 deposit for the card itself and then you can top it up with change at some machines or with a debit or credit amount at many others and in-person counters. You can either return the card and have the deposit amount and any outstanding balance returned, or you can keep the card for a future visit / as a souvenir.
The convenience is in being able to just swipe a contact card like local commuters do, which means there's a daily limit to how much you'll be charged; just remember that London's Tube system is metered by Zone and that your upper limit can change depending not on how many trips you take in a single day, but by the Zones in which you use your Oyster Card.