Public blockchains.

They are accessible to any user in the world who has a computer and an Internet connection. Undoubtedly, two of the examples of public blockchains are the aforementioned Bitcoin and Ethereum.

Private blockchains.

These are blockchains that are not open to the public, and can only be accessed by invitation. More recent than public blockchains, they tend to differ greatly between them. Among the most famous are Hyperledger, created by the Linux Foundation; Ripple, a protocol to facilitate international cash transfers, and R3, a consortium of banks that develops solutions applied to the financial sector.

Hybrid blockchains.

As the name suggests, it consists of a combination of public and private blockchains. In hybrid blockchains, participating nodes are invited, but all transactions are public in nature. This means that nodes contribute to the maintenance and security of the blockchain, although all transactions are visible to users around the world who don't have to know the content of the blockchain, unlike what happens with private blockchains. One of the most popular hybrid blockchains is BigChaindB, a kind of database with the characteristics of a blockchain.