The bitcoin network is a peer-to-peer payment network that operates on a cryptographic protocol. Users send and receive bitcoins, the units of currency, by broadcasting digitally signed messages to the network using bitcoin cryptocurrency wallet software. Transactions are recorded into a distributed, replicated public database known as the blockchain, with consensus achieved by a proof-of-work system called mining. Satoshi Nakamoto, the designer of bitcoin claimed that design and coding of bitcoin began in 2007. The project was released in 2009 as open source software.
Because it's similar to gold mining in that the bitcoins exist in the protocol's design (just as the gold exists underground), but they haven't been brought out into the light yet (just as the gold hasn't yet been dug up). The bitcoin protocol stipulates that 21 million bitcoins will exist at some point. What "miners" do is bring them out into the light, a few at a time.
The unit of account of the bitcoin system is a bitcoin. Ticker symbols used to represent bitcoin are BTC[b] and XBT.[c] Its Unicode character is ₿.:2 Small amounts of bitcoin used as alternative units are millibitcoin (mBTC), and satoshi (sat). Named in homage to bitcoin's creator, a satoshi is the smallest amount within bitcoin representing 0.00000001 bitcoins, one hundred millionth of a bitcoin. A millibitcoin equals 0.001 bitcoins, one thousandth of a bitcoin or 100,000 satoshis.