This tutorial describes the use of a "key" in many cryptographic functions and algorithms. You have probably also encountered the related concepts "passphrase" and "password" in various contexts. The differences are worth understanding.

Password and passphrase are terms with only a fuzzy boundary between them. In general, a passphrase is longer than a password, but particular descriptions may not make a precise distinction. Either a passphrase or a password is usually something an end user actually types into an interface to gain certain permissions or privileges, or to carry out specific restricted actions. The key used by the actual cryptographic algorithm is derived from the password or passphrase.