Blockchain is a distributed database that maintains a continuously growing list of data records secured from tampering. Blockchain is the main technology behind bitcoin, where it serves as the public ledger for bitcoin transactions. Many alternative coins (eg. altcoins) emerged after the rise of bitcoin. Blockchain technology is the backbone of all of them. After people started to understand how a blockchain works, they started using it for other purposes, like storing value, identities, agreements, property rights etc. Ethereum is the biggest innovation following the creation of bitcoin. It provides a way to create online markets and programmable transactions known as smart contracts. The three main types of blockchains that currently exist are:
Anyone in the world can send transactions to and expect to see them included if they are valid, and anyone in the world can participate in the consensus process. The consensus process determines what blocks get added to the chain and what the current state is. Public blockchains are open-source and everyone can be a part of them. Anyone in the world can explore the blockchain, send transactions or contracts, consult them and participate in the consensus proces.. Examples: Bitcoin, Ethereum Find a list of all publicly listed blockchains and their market cap here.
The consensus process is controlled by a pre-selected set of nodes for consortium blockchains. For example, one might imagine a consortium of 15 financial institutions, each of which operate a node. As part of this consortium, 10 must sign every block for the block to be valid. The right to read the blockchain may be public, or restricted to participants. Example: R3
Writing permissions are kept centralized to one organization. Read permissions may be public or restricted to an arbitrary extent. Likely applications include database management, auditing, and more that are internal in a single company. As a result, public readability may not be necessary in many cases at all, though in other cases it is desired. Examples: Eris Industries, Multichain