What is blockchain? First and foremost, Blockchain is a public electronic ledger — similar to a relational database — that can be openly shared among disparate users and that creates an unchangeable record of their transactions, each one time-stamped and linked to the previous one.
Each digital record or transaction in the thread is called a block (hence the name), and it allows either an open or controlled set of users to participate in the electronic ledger. Each block is linked to a specific participant.
Blockchain can only be updated by consensus between participants in the system, and when new data is entered, it can never be erased. The blockchain contains a true and verifiable record of each and every transaction ever made in the system. Source.