What is Bitcoin?
Bitcoin is a peer-to-peer payment system using the cognominal payment unit and transfer protocol. Cryptographic methods are used to ensure the operation and protection of the system. All information about transactions between addresses in the system is publicly available. Transactions are irreversible; electronic payment between the two parties occurs without intermediaries. However, a third party, guarantor, can be involved with the help of multi-signature. Nobody can freeze funds, even temporarily, except for the owner himself.
Read more about Bitcoin and its features on Wikipedia.