A public blockchain is a permissionless distributed ledger that is open to anyone. It allows anyone to become a node, write data to the blockchain, verify or validate information, and participate in the consensus process of the network. It is completely decentralized, meaning it does not require any third party or central authority for its functioning.
A private blockchain, on the other hand, is a permissioned distributed ledger. It requires digital identities and trust among participants to interact with the ledger. The participants must be invited and approved by a network administrator before they can join the network. Private blockchains also typically require less computing power and are faster than public blockchains.