-like systems, login
is a program responsible for authenticating
Ordinarily, when you sit down at a terminal of a Unix box, you will see a login: prompt. Confusingly enough, this isn't actually provided by the login program, but rather by getty, the daemon that awaits activity on an idle terminal. When you enter your username, getty runs login, which asks for your password and -- if you enter the right password -- sets up your environment and spawns your shell.
Depending on the setup of the system you're on, login may check your password against any of several different authentication sources. The default on most systems is the /etc/passwd file. Others may use LDAP, NIS, or even such oddities as NetInfo.