Distributed ledger technology (DLT) is a consensus of replicated, shared, and synchronized digital data geographically spread across multiple sites, countries, or institutions.   

Firstly, a key point is that data does not reside in a single, centralised database: instead, data is distributed across the nodes of the network.  A peer-to-peer network is required as well as algorithms that define a mutually agreed approach for validating and accepting data, to ensure that data can be replicated across the nodes of the network.
Secondly, there is not a single, centralised administrator; therefore, network participants have the ability to manage their own nodes.

Blockchain is a specific type of distributed ledger technology.