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Today people often use the word "layer" to indicate a <div> that is placed in a specific position on the page, even along the "z-index" [that is, apparently on top of, or beneath, other page elements.]
Positioning a div wherever you want on a page looks a lot like having different pieces of your page on transparent layers, and then overlaying the layers however and wherever you want. Using divs or "layers", it is very easy to create and maintain special styles inside each <div> or layer without affecting the appearnce of the rest of the page.
By adding JavaScript, div's can be made to appear, disappear, or move around the page in response to user actions, time elapsed, or other variables you choose. Some interesting animation-like effects can be created by these more advanced uses of layers. This technique of combining Cascading Style Sheets with JavaScript is called DHTML.