The terms blockchain and distributed ledger technology are often used interchangeably. However they are not the same.
A distributed ledger is essentially a database that is spread over several computing devices. These are called nodes.
Each note is updated independently. To reach consensus (a universal update throughout devices), various voting mechanisms are employed.
Hence, the revolutionary feature of a distributed ledger technology is absence of any central authority, this allows to reduce the cost of trust.
Blockchain is just one case of many in application of distributed ledger technology.
Blockchain, as the name suggests, is a chain of blocks. However not all distributed ledger systems organize data in the chain of blocks.
In blockchain, data is organized in blocks, they are connected to each other using cryptography, to secure the communication, including transactions.
Blockchain is a perpetually growing list of records, as nothing is allowed be to be deleted or changed. Any edit or removal is just another block.
This append-only structure is an optimal solution for recording events, managing records, processing transactions, tracing assets, and voting.
Every blockchain is a distributed ledger, but not every distributed ledger is a blockchain.
Both concepts use decentralization and consensus among nodes.
However, the blockchain organizes data in blocks, and updates the entries using an append-only structure.
Distributed ledgers broadly, and blockchains specifically, are conceptual breakthroughs in managing information and will be applied in every kind of human activity.