Bitcoin is a cryptocurrency invented in 2008 by an unknown person or group of people using the name Satosh Nakamoto, and started in 2009 when its implementation was released as open-source software.
It is a decentralized digital currency without a central bank or single administrator that can be sent from user to user on the peer-to-peer bitcoin network without the need for intermediaries. Transactions are verified by network nodes through cryptography and recorded in a public distributed ledger called a blockchain. Bitcoins are created as a reward for a process known as mining.
The birth of bitcoin in 2009 opened doors to investment opportunities in an entirely new kind of asset class - cryptocurrency. Consequently, the bull run of 2017 saw them become millionaires/ billionaires. Even those who didn't invest much reaped decent profits.