Blockchain 2.0
 A visual demonstration of the six key components in a blockchain
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A blockchain – originally block chain – is a distributed database that maintains a continuously growing list of records, called blocks, secured from tampering and revision. Each block contains a timestamp and a link to a previous block. By design, blockchains are inherently resistant to modification of the data — once recorded, the data in a block cannot be altered retroactively. Through the use of a peer-to-peer network and a distributed timestamping server, a blockchain database is managed autonomously. Blockchains are "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way. The ledger itself can also be programmed to trigger transactions automatically."
Blockchains are secure by design and an example of a distributed computing system with high byzantine fault tolerance. Decentralized consensus can therefore be achieved with a blockchain. This makes blockchains suitable for the recording of events, medical records, and other records management activities, identity management,transaction processing, and documenting provenance.
The first blockchain was conceptualised by Satoshi Nakamoto in 2008 and implemented the following year as a core component of the digital currency 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 use of a trusted authority or central server. The bitcoin design has been the inspiration for other applications.
The first work on a cryptographically secured chain of blocks was described in 1991 by Stuart Haber and W. Scott Stornetta, followed by publications in 1996 by Ross J. Anderson, in 1997 by Michael Doyle and 1998 by Bruce Schneier and John Kelsey. In parallel, Nick Szabo was working in 1998 on a mechanism for a decentralized digital currency that he called bit gold. In 2000 Stefan Konst published a general theory for cryptographic secured chains and suggested a set of solutions for implementation.
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Wikipedia