‘Ledgers’, or put simply, records of activity, were historically maintained on paper, more recently these were transferred to bytes on computers, and are now supported by algorithms in blockchains.
They are essentially an asset database that can be shared across a network of multiple sites, geographies or institutions. All participants within a network can have their own identical copy of the ledger. Any changes to the ledger are reflected in all copies in minutes, or in some cases, seconds. The assets can be financial, legal, physical or electronic.
The security and accuracy of the assets stored in the ledger are maintained cryptographically using ‘keys’ and signatures to control who can do what within the shared ledger. Entries can also be updated by one, some or all of the participants, according to rules agreed by the network. (Taken from UK Government: ‘Distributed Ledger Technology: beyond blockchain’).