Bitcoin is a decentralised digital currency, or cryptocurrency, with no central issuer or controlling authority.
It’s based on the ideas of an anonymous cryptographer called Satoshi Nakamoto, who published a whitepaper in 2008 detailing how it works. It was released as open-source code and the network went live in 2009, making it the first-ever cryptocurrency in existence.
The crypto- in cryptocurrency refers to the way the system employs cryptography and a robust network of computers to issue new units of the currency, maintain a public ledger of who owns what, confirm transactions and keep the network secure from misuse or attack.
Bit coin with a capital B refers to the Bitcoin network and all that goes into keeping it running. Bit-coin with a lower-case b refers to the bitcoin currency, which can be sent peer-to-peer anywhere in the world, almost instantly with negligible fees.