Click on that funny looking X thing above the padlock next to the desktop buttons on the panel to exit KDE. CTRL-ALT-BACKSPACE will also do the trick by killing the X server, but its use prevents session management. In addition, there's a logout option from the "K" button, the "Where do you want to go tomorrow?" one. Last but not least, if you right-click on an empty area of the desktop, you will be presented with a menu containing logout as one of the options.