A blockchain is a decentralized digital ledger. It’s decentralized because there is no single entity in charge of maintaining the ledger. Many parties, called validators, maintain their own copy of the ledger. The validators use a consensus protocol to ensure they always agree on the contents of the ledger.
A desirable property of blockchains is that validators agree on the contents of the ledger even in the presence of byzantine (malicious) validators who do not follow the rules of the consensus protocol.
A private blockchain is a permissioned blockchain. Private blockchains work based on admission controls which limit the people who can join in the network. There are one or more entities that regulate the network. (Blockchain Council, 2021)
A public blockchain is permissionless. Anyone can participate in this decentralized network. A public blockchain doesn’t have a single entity that controls the network. Data on a public blockchain is secure as it isn’t possible to change data once they have been validated on the blockchain. (Blockchain Council, 2021)