Historical version 1 of Travelcard (view current version)
Possibly the best value ticket option if you like to use a range of transport options. Travelcards are valid on the Tube, National Rail overground trains, trams and the DLR within the appropriate zones, and on buses throughout London.
The cost of a travelcard depends on how many zones you want it to cover. Essentially, if your zones include zone 1 then adding an extra zone or two isn't going to bump up the cost too much.
Day and weekend travelcards (and I think zone 1 or 1–2 weekly travelcards) can be bought from machines with no need for a photocard. For longer periods you will need to go to the ticket office and show your photocard.
Physically, the travelcard is just a credit-sized piece of thin plastic with a data strip on the back.
The Oyster Card has just been launched to the public after a period of trials with London Transport staff members. This is the high-tech replacement for the travelcard.