what is Cryptocurrency?

Cryptography (not cryptology, which contains cryptography and cryptanalysis) is a study of encryption from a mathematical point of view. It is also used for secret communication, which allows the world to see an encrypted message in case someone is listening, and does not let them know the real word. Only the intended recipient can read it. As you mentioned, you can also "register" things using a similar scheme, but where "pre-decrypt" (my term) is something, and other people who link your cryptographic key can show that you've signed something. So, cryptography is used almost exclusively in security areas. You can only restrict those authorized to access secret data. This is useful for sending messages, ensuring that only paid customers can install the software and be responsible (at least trying to prevent people from "joking" with their phone.