What is Blockchain?
Blockchain is a specific type of database made up of blocks of data, concatenated together to form a chain.
Specifically, new data comes in piece by piece in the form of transactions. Each transaction is added to a block. Each block is chained to the previous block, creating a chronological ordering of data.
While blockchains can store any type of data, the most common use is to produce a ledger, i.e., an append-only list of transactions. For example, Bitcoin's blockchain stores all the transactions from one wallet (account) to every other. The balance of each account is a reduce over these transactions.
Blockchains use cryptography and distributed computing algorithms to provide immutability, meaning that the data stored on the chain cannot be altered after the fact. For a distributed digital ledger (e.g. Bitcoin) this means that the transactions are permanently recorded and viewable to anyone.