What is blockchain?

Blockchain - a distributed ledger system. A sequence of blocks or units of digital information stored sequentially in a publicly accessible database. The foundation for cryptocurrencies.
Blockchains consist of a series of individual blocks. Each block contains information about transactions conducted over a specific period of time. They also contain a unique identifier to distinguish them from any other block in the chain. Blocks are created by solving cryptographic problems. The process of solving these problems is known as mining. Mining a block in a blockchain yields a reward. For example, during the creation of the Bitcoin blockchain, miners who solved the cryptographic hashing problem required to add a new block to the blockchain were rewarded with 50 BTC. Blockchains — are decentralized records. Instead of being stored in one central location, a blockchain is stored on the computers of every user of that blockchain.
Meanwhile, the unique identifier of a block, known as a hash, is derived from the information contained in each preceding block in the blockchain. This means that in order to falsify any record in the blockchain, an attacker would have to change every block in every instance of the blockchain. As a result, blockchains are considered practically unfalsifiable and are viewed as immutable records of transactions. To date, most blockchains are public. This includes well-known cryptocurrencies such as Bitcoin and Ethereum. Anyone can view the records of transactions conducted in a given blockchain using a tool called a block explorer. Theoretically, however, blockchains provide a high level of anonymity for users.
While public blockchains are the norm, private versions are also being explored as a solution for many use cases in business and government.