To give your page, or parts of your page, a Three-dimensional look, you can put a pattern in the background of your body, divisions, tables, or cells.
Normally, when you place a BACKGROUND image in the <BODY>, <TABLE> etc, the image tiles as normal. But sometimes, the colors of the image do not match the theme colors of your website. One way make sure that the colors all match is to set the theme colors in the BGCOLOR of the BODY, DIVISION, CELL etc. But then you get a solid area of color, and it frankly looks boring.
To keep the interest in that area, without losing the color matching, you can overlay a pattern on top of the correct color. Normally when you use a BACKGROUND image, it overrides and BGCOLOR you use. But if that BACKGROUND image has any transparent colors in it (available on in a .gif image), then the BGCOLOR bleeds through the transparency. The trick is to have the BACKGROUND image balance the amount of transparency with the right amount of pattern.
Below is an application that allows you to mix colors with patterns to give your page more interest.