A travelcard is a ticket for public transport in London, which allows the holder to use the following modes of transport within the zones on the ticket:
Valid cards also entitle the holder to discounts on river services.
Zones
Travelcards are valid for one or more of the ten zones. The six numbered zones are arranged concentrically, with Zone 1 including the central area, and Zone 6 including outlying towns such as Kingston, Uxbridge, Loughton and Upminster.
Travelcard's coverage is actually bigger than Greater London itself:
The Metropolitan Line of London Underground has seven stations outside Greater London which are not covered by the six numbered zones. These stations (Croxley, Watford, Rickmansworth, Chorleywood, Chalfont & Latimer, Amersham, Chesham) are arranged into four further zones (A, B, C and D), according to their remoteness from Central London.
The southwestern part of Epping Forest District (including Buckhurst Hill, Loughton, Debdon, Theydon Bois and Epping) of Essex is covered in Travelcard zones 5 or 6 instead of having special fares arranged.
Validity
One, three and seven day tickets can be purchased on a card with a magnetic strip or loaded onto an Oyster Card. A Travelcard lasting more than seven days can only be purchased on an Oyster Card.
Photocards are only required for travelcards of seven days or more which include travel on the National Rail network outside London. Special photocards are issued to those who receive a discount, such as students.
Your Travelcard will get you to every attraction covered by the London Pass, even Windsor Castle which is outside the normal Zones 1-6.
Your Travelcard will be date stamped on issue to reflect the days it is valid for.
If you are flying into one of these airports and have had your Travelcard delivered to you at home, you will be able to use it on the public transport services shown above, to get into the city centre.