A blockchain is a distributed, append-only database. That means users can edit it, but items cannot be deleted. It is distributed in the sense that the database is replicated on multiple entities, so that the data remains accessible even if some fraction of those entities crash or even if some of the entities are corrupted by an attacker. Data stored in the database can always be recovered at some later time. Many blockchains are also publicly accessible, in the sense that anyone can read the data stored theiren; however, this is not always the case. Blockchains are used for cryptocurrency transactions, and also have applications in the energy, healthcare and education sectors.