Wikipedia defines a distributed ledger as:
"A distributed ledger is a consensus of replicated, shared, and synchronized digital data geographically spread across multiple sites, countries, or institutions. There is no central administrator or centralised data storage."
"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. But not all distributed ledgers have to necessarily employ a chain of blocks to successfully provide secure and valid achievement of distributed consensus: a Blockchain is only one type of data structure considered to be a distributed ledger."
Fundamentally, there are three key aspects to a Distributed Ledger:
Blockchain networks can be private/permissioned or public/permissionless. Bitcoin and Ethereum operate on permissionless blockchain networks. In a permissionless blockchain, anyone can participate by reading data, submitting transactions, and participating in the validation process. In a permissioned blockchain, users set rules about participation, access, and the consensus mechanism.
Blockchain networks are made possible through the convergence of concepts from distributed algorithms, databases, and cryptography.
The following, taken from a Harvard Business Review article, very nicely describes the promise of blockchains:
"Contracts, transactions, and the records of them are among the defining structures in our economic, legal, and political systems. They protect assets and set organizational boundaries. They establish and verify identities and chronicle events. They govern interactions among nations, organizations, communities, and individuals. They guide managerial and social action. And yet these critical tools and the bureaucracies formed to manage them have not kept up with the economy’s digital transformation."
"Blockchain promises to solve this problem."
"Blockchain is a foundational technology: It has the potential to create new foundations for our economic and social systems."
There are a huge number of potential use cases of blockchains. Some of these uses cases are in supply chains and trade finance, cross border payments, various types of financial transactions, digital identities, document digitalisation, asset tracking, and smart contracts.
We are at an inflection point, where blockchain networks can enable transactions between enterprises and amongst individuals, which are not possible otherwise, or are currently inefficient.