A blockchain is a decentralized, digital ledger that records transactions. It is descentralized because the information is repeated in multiple computers. Transactions are recorded in blocks, which are connected sequentially in chronological order. Each block contains the list of transactions, a timestamp, and a reference to the previous block. Once data is added, it's nearly impossible to alter without changing all subsequent blocks and without changing the repeated information in multiple computers. This makes the blockchain highly secure.
This technology is mainly used in cryptocurrencies like Bitcoin but can also store other data types, supporting applications in fields like finance, supply chain management, and voting systems.