Some widgets are linked to larger apps. With the Address Book widget, for example, you can search for a name and address without opening the full Address Book app. Other widgets, such as Weather, provide information from the web without opening a web browser.
Dashboard is a full screen app that’s always available. To get there, just swipe up with three or four fingers, and then click Dashboard (first thumbnail on the far left next your desktop). Or press the Dashboard key on your keyboard. If your keyboard doesn’t have a Dashboard key, press F12 (Fn-F12 on portable keyboards).