Login process: UNIX systems run a process called login, to let the users get into the system. You should be aware that you may not be the only user working in that system. In order to be fair, do not misuse the system resources. Also when you leave the system (logout), remember that other people may continue to use that system. If you were sitting on the console (physical machine the system is working on, not a PC), do not shut down the system. UNIX machines are designed to be open 24 hours a day and 7 days a week. If you thing that your console is not working, call your system administrator.
When you decide to go, do not! close the screen and go away. You have to see the login screen to make sure that you have logged out. If you do not log out from the system, the next person sitting on the same machine can make everything to and from your account (you are responsible for the damage someone does from your account.