Transparency makes it possible to create nonrectangular images for the web. Background transparency preserves transparent pixels in the image. This allows the background of the web page to show through the transparent areas of your image. Background matting simulates transparency by filling or blending transparent pixels with a matte color that can match the web page background. Background matting works best if the web page background is a solid color and if you know what that color is.
Use the Transparency and Matte options in the Save For Web & Devices dialog box to specify how transparent pixels in GIF and PNG images are optimized.
To select a matte color, click the Matte color swatch and select a color in the color picker. Alternatively, select an option from the Matte menu: Eyedropper (to use the color in the eyedropper sample box), Foreground Color, Background Color, White, Black, or Other (to use the color picker).