When you are finished with the KDE you should log out so that other people cannot start using the computer with your security credentials (and possibly cause damage to the system).

To log out you can select the Logout command from the Application Launcher or the Logout button (with an X on the front) near the desktop buttons on the Panel. Either way you will be asked if you really want to log out. Just click Logout and you will be taken back to the log in screen.