A blockchain is a distributed database; its integrity is ensured by Cryptographic Hash functions; it can be public or private. Cryptographic Hash Functions are one-way functions that transform various inputs into a fixed length output. They are used for checking the integrity of files, protecting passwords, and for proof of work. A blockchain is stored on many nodes; once the data reaches a determined size, it is hashed, and its hash forms the first line of the next block. The advantages of the blockchains are they are immutable, data is not corrupted by the host computer, and they can provide good anonymity. The disadvantages are they consume many times more energy for storing the same quantity of data, and they are vulnerable to 51% Attacks. Blockchains can enhance cybersecurity to very high levels. They are and can be used in cryptocurrencies, smart contracts, transferring financial instruments, Distributed DNS, Public Key Infrastructure, Certifying Authorities, anonymizing data in health care industry, as repositories of public registers, IoT, traceability and supply chain management, public register of antiquities, etc. Blockchain is a paradigm shift in the way data is guarded and shared.