Blockchain is a decentralized, distributed and public digital ledger that can be used to track just about any type of data. The records in it cannot be altered easily.
Blockchain is decentralized so there is no one authority that maintains the ledger. This means that everyone can see and track movement and changes. When used for a cryptocurrency (like Bitcoin) it proves who owns the currency and how much is owned. When used for data, it can keep track of the true owner and who has rights to the data.
Blockchain uses cryptography and advanced mathematical principles together with data to achieve three objectives: (1.) secure transactions, (2.) control the creation of additional units of currency, and (3.) verify the transfer of value. It uses a peer-to-peer decentralized system to conduct transactions and protect the anonymity of users. The entire process is online.