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The approach needed is called DHTML - which is actually a combination of JavaScript and CSS. The basic idea is that a div somewhere on the page is given a display:hide rule. Then when a certain event occurs (onClick and onMouseover are the most common) the display rule is changed to display:visible.
If you're researching this, it's often called show/hide divs. It can get complicated depending on many factors on the page, but here's one thread we have on the topic:
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