When a person owns a bitcoin or a fraction of a bitcoin, they have a digital signature or key of that bitcoin. Half of the key is private and the second half is public and stored on the blockchain. When you request money or receive money, everybody on the network gets a copy of the requests. Before the completion of each transaction, the participants agree upon a price for the item. This is a key feature in crypto currencies: The users decide on the price of the items, not a third party. If the two users agree that two bitcoins are worth the price of a house, that will be the price of the transaction. Users can decide if they want their currency to represent kilowatts of energy, shares in a company or personal property.