The Bitcoin technology - the protocol and the cryptography - has a strong security track record, and the Bitcoin network is probably the biggest distributed computing project in the world. Bitcoin's most common vulnerability is in user error. Bitcoin wallet files that store the necessary private keys can be accidentally deleted, lost or stolen. This is pretty similar to physical cash stored in a digital form. Fortunately, users can employ sound security practices to protect their money or use service providers that offer good levels of security and insurance against theft or loss.
Source : Bitcoin.org