Encyclopædia Britannica, Inc.
Bitcoin relies on public-key cryptography, in which users have a public key that is available for everyone to see and a private key known only to their computers. In a Bitcoin transaction, users receiving Bitcoins send their public keys to users transferring the Bitcoins. Users transferring the coins sign with their private keys, and the transaction is then transmitted over the Bitcoin network. So that no Bitcoin can be spent more than once at the same time, the time and amount of each transaction is recorded in a ledger file that exists at each node of the network. The identities of the users remain relatively anonymous, but everyone can see that certain Bitcoins were transferred.
Transactions are put together in groups called blocks. The blocks are organized in a chronological sequence called the blockchain. Blocks are added to the chain using a mathematical process that makes it extremely difficult for an individual user to alter the blockchain.
The blockchain technology that underpins Bitcoin has attracted considerable attention, even from skeptics of Bitcoin, as a basis for allowing trustworthy record-keeping and commerce without a central authority. Blockchain technology is also critical to NFTs (non-fungible tokens).