Travelling using the Underground (the Tube) in London [duplicate]

Possible Duplicate:
London on public transport: what kind of tickets to use?

I will stay in London for only 2 days, but will travel using underground train (tube) quite often.

So my questions are:

Should I buy an Oyster card? What is the minimum deposit of Oyster? How long is the validity of the Oyster card?
Is there any card that I can buy that allow me to travel freely for 2 days without any extra charge? (Something like 15 pounds card that will expire within 24 hours, but can be used countless times)
Or anybody has any other alternative?

UPDATE : from the link that I duplicated, I found all of the info I needed in http://visitorshop.tfl.gov.uk/

Pictures about "Travelling using the Underground (the Tube) in London [duplicate]"

Travelling using the Underground (the Tube) in London [duplicate] - Content couple using ticket machine in underground

Travelling using the Underground (the Tube) in London [duplicate] - Crop smiling Asian female in white shirt using ticket vending machine with touch screen in underground

Travelling using the Underground (the Tube) in London [duplicate] - Crop unrecognizable male passenger in casual clothes with backpack inserting ticket in entrance machine of automated gate while entering modern subway station

What is the name of the card you can use to travel on the Tube today?

Oyster cards can be purchased from a number of different outlets in the London area: Ticket machines at London Underground stations, which accept banknotes, coins, and credit and debit cards.

What happens if you tap in and out at the same station?

Same station exits If you touch contactless (card or device) or an Oyster card on a yellow card reader when you enter a station, and then touch out to exit it within a short period of time, you'll be charged a same station exit: Between 0 - 2 minutes: a maximum fare.

How do you travel on the London Underground?

The cheapest way to travel is with an Oyster card. An Oyster card allows you to travel between all parts of London on the Underground, Trams (DLR), Overground, some river boats, Emirates Air Line, and the iconic red London buses.

We Followed A London Tube Driver For A Day — Here's What It's Like

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: Alessio Cesario, Samson Katt, Samson Katt, Samson Katt