A blockchain is the foundational data structure behind Bitcoin. As its name suggests, a blockchain is a list of blocks. Each of these blocks contains data. In Bitcoin’s case, each block contains transactions created when one party sends bitcoin to another.
The blockchain can be thought of as a digital ledger that keeps track of every account on the network. The entire blockchain is the book that stores the record of every transaction that has ever taken place on the network. Each block, then, is like a new page added to the ledger to update the state of accounts on the network. Bitcoin’s blockchain is public, and thousands of nodes store identical copies of it, making it a decentralized ledger.
One special property of a blockchain is that it is immutable. Once a block is added to the blockchain, it is difficult to alter it. As more and more blocks are added on top, changing the original block becomes ever more difficult until it is practically impossible.