A distributed ledger (also called a shared ledger or DLT) is a consensus of replicated, shared, and synchronized digital data geographically spread across multiple sites, countries, or institutions.
A blockchain, originally block chain, is a growing list of records, called blocks, that are linked using cryptography.
Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data (generally represented as a Merkle tree).
By design, a blockchain is resistant to modification of the data. It is "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way".
For use as a distributed ledger, a blockchain is typically managed by a peer-to-peer network collectively adhering to a protocol for inter-node communication and validating new blocks.
Once recorded, the data in any given block cannot be altered retroactively without alteration of all subsequent blocks, which requires consensus of the network majority.
A peer-to-peer network is required as well as consensus algorithms to ensure replication across nodes is undertaken.
One form of distributed ledger design is the blockchain system, which can be either public or private.