The question is misleading. Bitcoin doesn't have backing. But other things may be backed by bitcoin.
Exactly. My favorite answer is: The same thing gold is backed by.
Not exactly.
Bitcoin and gold are both backed by the same thing - their unparalleled utility as a medium of exchange satisfying Aristotle's prerequisites:
Aristotle's Qualities of a Good Money
By Gold Investing 101
About 2000 years ago Aristotle defined the characteristics of a good form of money. They were as follows:
1.) It must be
durable. Meaning it must stand the test of time and the elements. Money is a medium of exchange and a store of wealth so whatever form it takes, it must be able to handle the wear and tear of constant trading and transactions.
2.) It must be
portable. Meaning it should be practical in the sense that it holds a high amount of 'worth' relative to it's weight and size. In other words, it's "worth" must be very dense. Imagine if money was in the form of lead bricks, these bricks would be very dense, but it would be a nightmare and near impossible to constantly exchange large amounts. And you can forget about carrying them around in your pockets.
3.) It must be
divisible and consistent. Meaning it should be relatively easy to separate and distribute in smaller forms without affecting it's fundamental characteristics. This concept also works in reverse in that it should be relatively easy to re-combine several divided pieces of the money into a larger, single piece. This makes houses and paintings and cars unpractical as forms of money because taking them apart would affect their fundamental characteristics. An extension of this idea is that the item should be 'fungible'. Dictionary.com describes fungible as:
"(esp. of goods) being of such nature or kind as to be freely exchangeable or replaceable, in whole or in part, for another of like nature or kind."
4.) It must have
intrinsic value. This characteristic carries a bit of a subjective quality in that everyone views the world through a different lens and what I view as valuable may not necessarily be valuable to my neighbor, but for the sake of argument let's just say that there is a consensus of value given to a certain material. The basic understanding behind intrinsic value is that the material carries 'worth' in and of itself. It does not derive it's value from anything else. It just sits there and is valuable. This is why paper currencies with no backing will not stand the test of time. Paper currencies only derive their "value" from what is known as legal tender laws, which are in essence a threat of legal prosecution, and or force, if they are not accepted as money for payment.
This fourth point brings up the point of
scarcity, which is in essence a matter of intrinsic value. Paper currencies in circulation today, such as the dollar, euro, yen, swiss francs, zimbabwe dollars, etc... they are all now purely fiat instruments. (by fiat, I mean that their use is declared by decree and usually by threat of force. Definition of fiat.) The governments that sponsor them have essentially unlimited power in their ability to create new supplies. Because of technology, it is now simply a matter of typing something into a computer and the amounts are instantly credited somewhere. So in theory the supply of dollars for instance is infinite, and it seems like lately the wizards in Washington are trying to see whether this theoretical limit can be reached. Take Zimbabwe as a practical real world example. It now takes trillions of Zimbabwe dollars to buy a roll of toilet paper.
http://www.gold-speculator.com/gold-investing-101/4476-aristotles-qualities-good-money.html