Usually HTML pages can be considered two-dimensional, because text, images and other elements are arranged on the page without overlapping. There is a single rendering flow, and all elements are aware of the space taken by others.
In CSS 2.1, each box has a position in three dimensions. In addition to their horizontal and vertical positions, boxes lie along a “z-axis” and are formatted one on top of the other. Z-axis positions are particularly relevant when boxes overlap visually. Learn how to control z-index in your CSS files…