Tabs suggest a physical space. Tabs create the illusion that the active tab physically moves to the front. It’s a cheap trick, but effective because it’s based on a visual cue. Somehow, the result is a stronger-than-usual sense that the site is divided into sections and that you’re in one of the sections.
Don’t make me think
Being an expert in online usability Krug gathered his experience in a wonderful book. This summary of key thoughts will help you to remember most important principles and laws.