Blockchains can be categorized into three main types based on their accessibility and control: Public, Private, and Consortium (also known as Federated or Hybrid) blockchains.

1. Public Blockchain

A public blockchain is an open and decentralized network that allows anyone to join, participate, and validate transactions. These blockchains are typically permissionless, meaning that anyone can become a node in the network and contribute to the consensus process. Bitcoin and Ethereum are examples of popular public blockchains. They are often used for cryptocurrencies and decentralized applications (dApps) that require a high degree of transparency and security.

2. Private Blockchain

A private blockchain, as the name suggests, is a blockchain network that is limited to a specific group of participants. It is also referred to as a permissioned blockchain because participants require permission or invitation to join the network. Private blockchains are often used in enterprise settings, where companies want to maintain control over who can participate and access the data. These blockchains might sacrifice some decentralization for increased scalability and faster transaction processing. Hyperledger Fabric is an example of a private blockchain framework.

3. Consortium Blockchain (Federated or Hybrid Blockchain)

A consortium blockchain is a combination of public and private blockchains. It is controlled by a group of organizations, often referred to as a consortium, rather than a single entity. Consortium blockchains aim to achieve a balance between the openness of public blockchains and the controlled access of private blockchains. These blockchains are typically used in scenarios where multiple organizations need to collaborate while maintaining a certain level of trust and privacy. R3 Corda is an example of a consortium blockchain platform. In summary, these three types of blockchains cater to different use cases and requirements. Public blockchains prioritize openness and decentralization, private blockchains focus on controlled access and scalability, and consortium blockchains offer a middle ground for collaborative efforts among multiple entities.