In UNIX (or at least in Linux, manpage says this is a BSD feature), the program that's responsible for logging in. init runs getty on a tty; getty gives you login; login gives you shell if you log in correctly.
login is used when signing onto a system. It can also be used to switch from one user to another at any time (most modern shells have support for this feature built into them, however). - Linux login(1) man page