A distributed ledger (also called a shared ledger, or Distributed Ledger Technology, DLT) is a consensus of replicated, shared, and synchronized digital data geographically spread across multiple sites, countries, or institutions. There is no central administrator or centralized 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.
The distributed ledger database is spread across several nodes (devices) on a peer-to-peer network, where each replicates and saves an identical copy of the ledger and updates itself independently. The primary advantage is the lack of central authority. When a ledger update happens, each node constructs the new transaction, and then the nodes vote by consensus algorithm on which copy is correct. Once a consensus has been determined, all the other nodes update themselves with the new, correct copy of the ledger.  Security is accomplished through cryptographic keys and signatures. 
Distributed ledgers may be permissioned or permissionless regarding if anyone or only approved people can run a node to validate transactions. They also vary between the consensus algorithm. (Proof of Work, Proof of Stake, or Voting systems). They may also be mineable (you can claim ownership of new coins contributing with a node) or not mineable (the creator of the cryptocurrency owns all at the beginning).
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