To clarify JohnnyColombia's advice:
an Oyster card is a contactless smartcard which can hold a balance (to use to pay as you go) and/or a season ticket valid for certain fare zones for a week or more.
You can also pay for individual journeys by cash (but it's more expensive), or buy a season ticket for a day, a week, or more, as a paper ticket.
It sounds like, for your purposes, a Travelcard (season ticket) on Oyster is your best bet - and, if you're getting one from a tube station, they'll most likely issue it on Oyster anyway. The card itself has a £5 refundable deposit.
Load the card with a Travelcard to cover the your regular journeys (eg if you're staying in zone 3 and working in zone 1, you'll need a z1-3 travelcard. Note that if your commute is z2 - z2 but via z1, you'll be charged for the journey via z1).
It is, in theory, possible to get a 10-week Travelcard (or, indeed, any length of time between 1 month and 1 year); I'm not sure how the pricing works, or if it would save you money, or merely confuse the ticket office clerk!
This pdf mentions 'odd period Travelcards', and that they're available at Tube and Overground ticket offices.
The Travelcard is valid on the tube, trains, and DLR within the zones you've paid for; and buses and trams all over London. To use services outside the zones, you'll need some pre-pay balance on the card, and it'll only charge you for what you've not already paid for. You can also use the pre-pay balance to pay for some river services.
If you have any pre-pay balance left on your card when you're leaving London, you can get it refunded along with the deposit at a tube ticket office.