Bitcoin is a digital currency. You can pay with it on spot, in online stores and make transfers. Although, Bitcoin is different than ordinary currencies you know. Advanced math (cryptography) secures each transaction in Bitcoin. It guarantees that transactions cannot be forged and the currency cannot be stolen - of course if the Bitcoin owner uses standard safety procedures (eg. doesn't pass his private key to third-parties).

Double-spending represents one of the most common risks regarding cryptocurrency transactions.