What is Bitcoin, the network?
The Bitcoin network is a trustless peer-to-peer network to send and receive bitcoins, the cryptocurrency, and where the transactions are recorded on the public ledger or database called the blockchain. It was invented by Satoshi Nakamoto.
Bitcoin is open and decentralized; anyone can run a node on the Bitcoin network and do work to mine bitcoins. A node can join and leave the network whenever they want. The number of active nodes at any given time therefore fluctuates.
Finalizing transactions on the network requires consensus among the nodes. Consensus on the state of the public ledger is reached through a process called bitcoin mining that uses a proof-of-work system. Proof-of-work is needed to add a new block to the bitcoin blockchain. The public blockchain allows the Bitcoin network to thwart a previously tricky problem with digital payments: double spending.