The term blockchain describes a technological breakthrough - not only in the field of digital currencies. However, the concept behind this term is not easy to grasp. This pocketbook supports a fundamental understanding. Blockchain is a technology that ensures security against certain attacks. Its significance only becomes clear when one has thoughtfully considered these attacks. After an explanation of the cryptographic foundations, the structure of a blockchain protocol is then traced. Starting from a protocol that anyone who has used e-banking is familiar with, problems are identified and solved step by step, leading to the development of new protocols - culminating in a blockchain. With this understanding, use cases for blockchains are identified, along with selected blockchain applications such as timestamps for documents.