What is Blockchain?

Blockchain is a technology that emerged with the creation of Bitcoin by Satoshi Nakamoto in 2009. The blockchain is the underlying technology allowing Bitcoin to work without any need of a trusted third party and by relying only on the protocol. Blockchains use a decentralized peer-to-peer network in order to achieve consensus and byzantine fault tolerance, on Bitcoin and Ethereum - which are the most used blockchains - this consensus is achieved by proof-of-work which provides a probabilistic solution to the byzantine generals problem.
The blockchain can be seen as a big ledger, containing all the past transactions and consultable by anyone as it is distributed. These transactions are contained in "blocks", each block can contain a finite number of transactions and the average time needed to create a new block is fixed in the implementation level of the blockchain. As each block is linked to the previous one by a cryptographical proof (the block header of a new block contains the hash of the previous block) the transactions that are processed in a block on the blockchain are immutable, this characteristic is necessary when it comes to the use of this technology for transferring value.