The blockchain is a distributed database system, which originated from Bitcoin. It was first proposed by Satoshi Nakamoto in 2008 in his paper Bitcoin: A Peer-to-Peer Electronic Cash System (Chinese Version). The link of the paper in pdf is provided by the official website.
The blockchain is a distributed ledger which connects data blocks according to the time sequence. Cryptography methods are adopted to ensure that a blockchain can’t be tampered or forged.
The blockchain is divided into public, consortium and private blockchains.
A blockchain on which everyone can read and send data, confirm transactions, and compete in the mining process.
Unless mastering over 51% of the hash power, no individual or institution can control or tamper with data. As a result, the "public blockchain" is considered to be "completely decentralized".
A public blockchain generally encourages the nodes to compete in the mining process with token incentives. BTC and ETH are typical public blockchains.
A blockchain on which the management is participated by various institutions, with each institution operating one or more nodes and recording transaction data in collaboration. The data can only be read, written, and transferred among institutions within the system.
The data on the blockchain is partially distributed, and we regard them as "multi-centralized". Typical examples include R3, Hyperledger, FISCO and so on.
Each node involved in the system needs to be permitted (such as users of specific institutions), and data access and use are under strict permission management. Some permissioned blockchains do not have a token mechanism because they don’t need to encourage nodes to compete in the mining process with tokens.