Blockchain is an example of a distributed public ledger; that is, a shared record system for transactions. It’s been described as “a technology that allows people who don’t know each other to trust a shared record of events”.
The idea is that every party involved in a particular type of transaction holds a copy of the entire ledger; there are no centralized databases. Anyone can enter a transaction onto the system, and at regular intervals these transactions are batched together into “blocks.” The blocks are then formed into “chains” (hence the name) using cryptographic technology that provides high levels of security. The chronological chain of transactional information is created in such a way that each block that is added protects the information in the previous one.
Source: A beginner’s guide to blockchain by Gemalto