In a consortium blockchain, the consensus process is controlled by a pre-selected group – a group of corporations, for example. The right to read the blockchain and submit transactions to it may be public or restricted to participants. Consortium blockchains are considered to be “permissioned blockchains” and are best suited for use in business.
Semi-private blockchains are run by a single company that grants access to any user who satisfies pre-established criteria. Although not truly decentralized, this type of permissioned blockchain is appealing for business-to-business use cases and government applications.
Private blockchains are controlled by a single organization that determines who can read it, submit transactions to it, and participate in the consensus process. Since they are 100% centralized, private blockchains are useful as sandbox environments, but not for actual production.
Anyone can read a public blockchain, send transactions to it, or participate in the consensus process. They are considered to be “permissionless.” Every transaction is public, and users can remain anonymous. Bitcoin and Ethereum are prominent examples of public blockchains.