Eli Dourado has a well written and easy to understand article about how Bitcoin transaction reversal and arbitration works. The feature built into Bitcoin is known as m-of-n or “multisignature” transactions. Here’s a brief extract:

The simplest variant is a 2-of-3 transaction. Let’s say that I want to buy goods online from an anonymous counterparty. I transfer money to an address jointly controlled by me, the counterparty, and a third-party arbitrator (maybe even Amex). If I get the goods, they are acceptable, and I am honest, I sign the money away to the seller. The seller also signs, and since 2 out of 3 of us have signed, he receives his money. If there is a problem with the goods or if I am dishonest, I sign the bitcoins back to myself and appeal to the arbitrator. The arbitrator, like a credit card company, will do an investigation, make a ruling, and either agree to transfer the funds back to me or to the merchant; again, 2 of 3 parties must agree to transfer the funds. This is not an escrow service; at no point can the arbitrator abscond with the funds.

The full article is on Eli’s blog.

Another feature of the currency that adds intrinsic value, along with no intermediaries, very fast transaction time and negative inflation which makes it a great value store.