When the host computer starts up it runs the program getty. The getty program runs the "login" program to log people in. See Getty (used in /etc/inittab). A "login:" prompt appears on the screen. People at the terminals log in (after giving their passwords) and then have access to the computer. When it's time to shut the terminal down, one normally logs out and turns the terminal off. See Login Restrictions regarding restricting logins (including allowing the root user to log in at terminal).