Any transaction issued with Bitcoin cannot be reversed, they can only be refunded by the person receiving the funds. That means you should take care to do business with people and organizations you know and trust, or who have an established reputation. For their part, businesses need to keep control of the payment requests they are displaying to their customers.

Bitcoin can detect typos and usually won't let you send money to an invalid address by mistake. Additional services might exist in the future to provide more choice and protection for the consumer.

More detailed explanation:

One of the fundamental innovations of Bitcoin is that a bank, or other trusted third party, does not need to arbitrate transactions. The Bitcoin protocol allows two people to exchange value, without ever knowing or trusting one another. This leads to increased transaction speed, fee-free transactions, and secure payments. It's revolutionary.

The Bitcoin network maintains a ledger of all transactions on the network - the parallel to this would be a bank's ledger which records payments sent to and from all the accounts within the bank. This ledger on the Bitcoin network is known as the blockchain. A bitcoin is essentially a spot on this ledger and "ownership" of a bitcoin means you own access to that spot on the ledger - the Bitcoin address.

Once a Bitcoin transaction is sent to the network you are transferring ownership of that spot on the blockchain - the recipient will then be the only person who can access that spot. Once the ownership of the address is changed, it is impossible to revert. This ensures that there is no risk involved when receiving Bitcoin and contributes to the speed and security of transactions. If you have mistakenly sent funds, you should contact the recipient and request a new transaction to send the funds back to your account.