At its most basic level, a blockchain is a type of shared database that differs from a typical database. It stores information; blockchains store data in blocks linked together via cryptography.
As new data comes in, it goes into a new block. Once the block fills up with data, it gets chained onto the previous block, which makes the data chained together in chronological order.
The original purpose of the blockchain was to facilitate digital transactions using Bitcoin, but the potential uses of blockchain technology quickly became apparent to developers and entrepreneurs in other industries.