Blockchain is a decentralized general ledger. It is a computer network that has an identical copy of the database and changes its status (records) to a common agreement based on pure mathematics. All this means that there is no need for any central server or any entity we trust (e.g. central bank, notaries, banks, Paypal, etc.). The blockchain is the technological basis for all cryptocurrencies, whether Bitcoin, Ethereum, Hyperledger, etc.