A blockchain is a transactional database distributed among all participants, without the need to trust each other through a central administrator. Can be accessed but not edited by all. Its the book of the truth.
Basically, a blockchain is a permanent inmutable record to exchange data, money, goods, and services. Its independently operated by Internet users and its distributed among their computer network.
BCs constrast with traditional databases: SQL, NoSQL because these are controlled by a single entity, even when having some distributed architecture.
Blockchains work like a ledgers but not all ledgers are blockchains. Criptocoins and digital coins are not the same either. A cryptocurrency is a pseudo-coin like BTC. A digital coin could be Eur, Usd, etc. or a private share ledger.