A distributed ledger is a ledger, a record of who owns what and of every transaction, that is copied and kept in step across many computers, called nodes, instead of being held by one organisation. The nodes agree on updates through a consensus process. The technology is explained in full under Distributed Ledger Technology (DLT).
A blockchain is the best-known kind of distributed ledger, but others, such as DAG-based ledgers, do not group records into blocks. The contrast is a central ledger, kept by a single institution such as a bank.