To understand Blockchain lets take an example, in our day to day value transfer we need a third party which takes care of Centralizing all the transactions for e.g banks for money, notary for land transfer etc maintaining a ledger.
This Centralization concentrates power in hands of few individuals. What if we return power back to people by Decentralizing it?
In November 2008, Satoshi Nakamoto published a Whitepaper for a new monetary system which,
This monetary system maintained a Digital Ledger on peer to peer network of nodes (individual computers) having following characteristics:
A Ledger can be understood as an ordered list of transactions meant for book keeping. When we maintain that bookkeeping online it is termed as Digital Ledger. A simple ledger is shown below maintaining records of all the value transfers between multiple parties,
Blockchain can be understood as a code that runs on the computers of all the participants in the trustless network.