The Blockchain is a new type of database. Instead of a centralised or decentralised database on one or more servers, a blockchain database is installed in individual computers used by the people who use the database. A copy of the blockchain database is installed on every computer of every user who uses the database. A distributed blockchain database is copied to every client computer on the network. There are no database servers.
When a transaction takes place in the blockchain database, a transaction takes place and is updated on the first client computer blockchain database, and the database transmits the transaction to every identical database on the distributed network over the internet. Then all the databases have to agree to the database update. We can think of this as consensus-based permission because all databases have to reach an agreement before the transaction is allowed. Each blockchain database then appends a new block of data containing the transaction.
Suppose somebody wants to hack the blockchain, by replacing the transaction. The blockchain database will not permit this because each row in the blockchain database relies on the previous row in the database so the integrity of the database cannot be broken. Database blocks/rows are only added and never deleted. All changes are captured as new blocks.
All blocks are read-only, we call this characteristic immutability. A blockchain database is an immutable database. A blockchain database keeps track of changes, just like an accounting ledger. This is why a blockchain is often referred to as a distributed ledger.