Use this tag for questions relating to authentication mechanism (i.e. proving to the system that you are you, using passwords, keycards etc.). In general don't use this and the security tag on the same question, unless it is about the security of the authentication process.
Authentication methods are used to assure that a user has a claimed identity. These methods can be based on knowledge (e.g. a password or a key), on a token (e.g. a smart card), or on biometrics (e.g. a human fingerprint). Different methods can be combined to strengthen the assurance. If two methods are combined, this is called two-factor authentication, e.g. smart cards typically need to be unlocked by a PIN.
Use this tag if your question is primarily concerned about authentication mechanisms. If you tag your question with authentication, then avoid the tag security unless your question is about the security of a particular mechanism.