A blockchain – originally block chain – is a continuously growing list of records, called blocks, which are linked and secured using cryptography. Each block typically contains a hash pointer as a link to a previous block, a timestamp and transaction data. By design, blockchains are inherently resistant to modification of the data. Harvard Business Review defines it as "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way." For use as a distributed ledger, a blockchain is typically managed by a peer-to-peer network collectively adhering to a protocol for validating new blocks. Once recorded, the data in any given block cannot be altered retroactively without the alteration of all subsequent blocks, which requires collusion of the network majority.
Blockchains are secure by design and are an example of a distributed computing system with high Byzantine fault tolerance. Decentralized consensus has therefore been achieved with a blockchain. This makes blockchains potentially suitable for the recording of events, medical records, and other records management activities, such as identity management, transaction processing, documenting provenance, or food traceability.
The first blockchain was conceptualized in 2008 by an anonymous person or group known as Satoshi Nakamoto and implemented in 2009 as a core component of bitcoin where it serves as the public ledger for all transactions. The invention of the blockchain for bitcoin made it the first digital currency to solve the double spending problem without the need of a trusted authority or central server. The bitcoin design has been the inspiration for other applications.
Alternative blockchains, also known as altchains, are based on bitcoin technology in concept and/or code. The term encompasses all blockchains but bitcoin's main chain. Compared to bitcoin, these designs generally add functionality to the blockchain design. Altchains can provide solutions, including other digital currencies, though tokens in these designs are not always considered as such. Altchains target performance, anonymity, storage and applications such as smart contracts. Starting with a strong focus on financial applications, blockchain technology is extending to activities including decentralized applications and collaborative organizations that eliminate a middleman.
Notable non-cryptocurrency designs include:
LaZooz — decentralized real-time ride sharing
Swarm and Koinify — decentralized crowdfunding
Steemitcombines a blogging site/social networking website and a cryptocurrency
Hyperledger— cross-industry collaborative effort from the Linux Foundation to support blockchain-based distributed ledgers. Most notable projects under this initiative includes Hyperledger Burrow (by Monax) and Hyperledger Fabric (spearheaded by IBM).
Counterparty— open source financial platform for creating peer-to-peer financial applications on the bitcoin blockchain
Bitcache
Bitnationis the world's first operational Decentralized Borderless Voluntary Nation, a Blockchain Powered Jurisdiction.
JPMorgan Chase's Quorum permissionable private blockchain with private store for smart contracts
Ethereumis a blockchain, with a Turing complete scripting language that enables the processing of smart-contracts on the blockchain.
Zero Knowledge Blockchain: a platform by QED-it, combining the advantages of coordinating the operative parties (blockchain technology) to the necessities of non data disclosure (ZKP) for legal or business motives (Zero Knowledge proof cryptology).
Source: Wikipedia
