‘just’ a ledger, keeping track of transactions?
What makes a blockchain special: the ledger is not being managed by a single centralized institution, such as a bank or government, but it is stored (as full copies) on many independent computers within a decentralized network. No single computer controls the ledger. Any computer on the network can make a change to the ledger, by following rules dictated by a “consensus protocol”. The consensus protocol is a mathematical algorithm that requires a majority of the other computers on the network to agree with the change.
Once a consensus generated by that algorithm has been achieved, all the computers on the network update their copies of the ledger. If any of them tries to add an entry to the ledger without consensus, or to change an entry retroactively, the rest of the network automatically rejects the entry as invalid.
It is called a block because several transactions are bundled together into blocks of a certain size. Then these blocks are chained together (hence “blockchain”) by cryptographic locks (a product of the consensus algorithm). This produces an immutable, shared record of the “truth,” one that cannot be tampered with (if things are done right).