Public blockchain is the model of Bitcoin, Ethereum, and Litecoin and is essentially considered to be the original distributed ledger structure.
This type of blockchain is completely open and anyone can join and participate in the network. It can receive and send transactions from anybody in the world, and can also be audited by anyone who is in the system.
Each node (a computer connected to the network) has as much transmission and power as any other, making public blockchains not only decentralized, but fully distributed, as well.
In order for a transaction to be considered valid, it must be authorized by each of its constituent nodes through the consensus process. Once this authorization takes place, the record is added to the chain. Public blockchains typically have incentives to encourage people to join the network as well as to authenticate transactions.
One of the biggest disadvantages of a public blockchain is its complete openness. This type of transparency implies little to no privacy for transactions and supports a weak concept of security. Another drawback is the substantial amount of computing power that is necessary for the maintenance of the ledger. With so many nodes and transactions as part of the network, this type of scale requires extensive effort to achieve consensus.