At the heart of bitcoin is a fundamental innovation: a distributed public ledger. A ledger in accounting is a book that you cannot edit once you have written in it. Instead, if you have made a mistake, the only way to fix it is to add another transaction to the ledger that undoes the error. As we know from accounting fraud, problems arise when people figure out ways to transact without recording it in the ledger or making ex post changes to the ledger (this is why Quickbooks isn't really an accounting system). The bitcoin ledger is the so-called blockchain which uses the fact that there are many copies of it that are broadly distributed combined with a fair bit of math to ensure that once a transaction has been recorded in the blockchain that transaction can not be changed after the fact. There is no other widely used protocol in the world today that accomplishes this: with bitcoin anyone can make a statement (a transaction) and have this be recorded in a globally visible and fixed ledger.

http://www.usv.com/posts/bitcoin-as-protocol