For one, proof of work prevents miners from creating bitcoins out of thin air: they must burn real energy to earn them. And two, proof of work ossifies Bitcoin’s history. If an attacker were to try and change a transaction that happened in the past, that attacker would have to redo all of the work that has been done since to catch up and establish the longest chain. This is practically impossible and is why miners are said to “secure” the Bitcoin network.
Transactions are verified by network nodes through cryptography and recorded in a public distributed ledger called a blockchain. Bitcoin was invented by an unknown person or group of people using the name Satoshi Nakamoto and released as open-source software in 2009. Bitcoins are created as a reward for a process known as mining. They can be exchanged for other currencies, products, and services. Research produced by the University of Cambridge estimates that in 2017, there were 2.9 to 5.8 million unique users using a cryptocurrency wallet, most of them using bitcoin.