Types of Blockchain

Public Blockchain: Bitcoin, presumably the most popular Blockchain in existence today is a Public Blockchain. What does it mean? A public Blockchain is a blockchain that can be accessed by anyone (most often anonymously). There are no restrictions on who can join and what transaction they can post as long as the transactions are mathematically valid. However be aware, that although members can join the network anonymously (revealing only their public key) , every transaction that they undertake are visible to everyone (Public), which can be carefully studied to identify the users.

In such a network there is typically an incentive given to the participants for executing a computing resource intensive consensus protocol (eg: validate a block using Proof-of-work)

Permission Blockchain: A permissioned Blockchain, is one in which the interaction within the Business (Blockchain) Network is restricted to users who have access rights provided by the network owner/s. In such a network, non-anonymous validation of Blocks or interaction with the Blockchain is not permitted. Usually a Certificate Authority (CA) is used to manage access to such a network. A Blockchain platform (say Ripple) running its network as a permissioned network, will determine who are allowed to be validators and what privileges are given to what users. Hyperledger Fabric is one of the most prominent example of a permissioned Blockchain framework.

In such a network it is possible to assume that a transaction block is valid based on whether it has been generated by someone who has the correct privilege. Such a Blockchain deployment, where there is no mining, can be argued to be not a Blockchain at all and only a shared ledger. Let's not get into the debate now.

Consortium Blockchain: It is possible that a single (originating) organisation will maintain the Blockchain (centralized) and provide predefined access rights to interacting parties. Such a network typically suits government or regulatory bodies who have legal purview over other participants. It is again debatable whether to call such a network, a Blockchain network, as the ledgers are stored centrally and are not distributed among participating nodes.