Authentication is related to the mechanisms used to ensure that the user is who they say they are and operates at two key levels, namely, local and remote.
Authentication can take various forms, the most common is user login, but other examples include fingerprint reading, iris scanning, and PIN number entry. User logins can be managed on a local basis, as you would on your personal computer, for example, or on a remote basis using a tool such as Lightweight Directory Access Protocol (LDAP). Managing users remotely provides roaming user profiles that are independent of any particular hardware and can be managed independently of the user. All of these methods execute at the operating system level. There are other mechanisms that sit at the application layer and provide authentication for services, such as Google OAuth.
Alternative authentication methods have their own pros and cons, a particular implementation should be understood thoroughly before declaring...