The Photoshop interface can get to be quite cluttered with toolbars, histograms, actions, layers, etc. Every one of these things decreases your available viewing area for your photo, making it harder to work with. So when you need more room, press the “TAB” key — it hides the toolbars and give you a nice (nearly) full-screen view of your image. This is effective for things like fitting the image to the screen or working at 100% magnification. The newer versions of Photoshop will temporarily bring back a toolbar when you mouseover the sides of the screen — which will allow you to change tools and such. If this doesn’t work for you, just press the “TAB” key again and your toolbars will reappear. Ta da!