A blockchain is a chain of blocks containing data, where each new block holds a cryptographic reference (hash) to the previous one. This makes it practically impossible to alter a record after the fact: you would have to rewrite all subsequent blocks on the majority of network nodes.
Copies of the ledger are stored on thousands of computers around the world, so a blockchain has no single point of failure and no single owner. This is the technology that powers Bitcoin, Ethereum and most cryptocurrencies.