I think I've read so much about these that I've actually gotten myself confused. Please help. I'm going to London for 6 full days and will be travelling quite a bit in zones 1-6. What's the difference between an Oyster Card and say, a 7-day Travelcard? Which would be best?
An Oyster card is simply a vehicle for storing tickets. You can store a weekly travelcard, pay-as-you-go credit, or both on a regular Oyster card. Are you staying within zone 1-2? If I were going to be in London for 6 days, I would put a 7-day zone 1-2 card on an Oyster along with 10 GBP of pay as you go money. The pay as you go money would cover the cost of trips from the border of zone 2 to the outer zones. You could add more if you need to later. If you are traveling solo, I would go with an Oyster card for convenience. If you are travelling with another person, then the paper travelcard sold by the railway is another option that could save you some money. There are some 2 for 1 offers available to rail ticket holders. To be eligible for this, you must buy the travel card at a rail station ticket office (Paddington, Victoria, etc.) and not from the tube station. The places with the 2 for one offers are described here: http://daysoutguide.co.uk/2for1-london
As Laura says for most people who are travelling 5 or more days out of a 7 day period a 7 day Travelcard is better than an Oyster Pay As You Go and for visitors the number of times they venture outside the central zones 1 & 2 doesn't normally warrant buying coverage of zones 1 to 6 just for an odd trip to Heathrow or Hampton Court. If you are staying in the suburbs its a different matter. The other major complication is if you have kids. Anyone 18 or under needs an Oyster ID Card applied for at least 3 weeks in advance to get child fares on both Oyster and Travelcard and costs 10 GBP. The main workaround that doesn't need the id card (for under 16's at least) is the 1 day child Travelcard, the 7 day Travelcard and Oyster Cards on Pay As You Go need the id card.
Kids under 11 travel free on bus and Underground