The matte is the background colour you want to use for your image. If you have selected a dithering method (see above), this option lets you control how much the image is dithered – 0 means no dithering, 100 means lots of dithering. If your image has very few colours in to start with, or doesn’t look too bad with fewer colours, select 16, 8, 4 or even 2! This will make the GIF file size smaller. A palette of 32 colours is often sufficient for web images, but if your image has a lot of detail and looks too fuzzy/blurry/banded with 32, up it to 64, 128 or 256. This is where you choose the size of your GIF palette. Use only if you don’t mind reducing the image quality quite severely! A value of 0 will not remove any detail a value of 100 will remove the maximum amount of detail. This slider allows you to remove some detail from the image, in order to reduce the file size further. If enabled, the GIF will appear gradually as interlaced horizontal lines as it is loaded on the Web page, which gives viewers something to look at while the full image appears. If you deselect this checkbox, the transparent areas will instead be filled with the Matte colour (or white if no matte is selected). It specifies that you want parts of the saved GIF to be transparent. This checkbox is only available if your image does not have its Background layer turned on. Noise: Similar to Diffusion, but does not produce seams at the edges of slices in ImageReady.
Can work well for some images, but often produces a rather artificial look to the image. Pattern: Uses a pattern for dithering, rather than random dots.Can produce seams when using ImageReady slices. Diffusion: Uses error diffusion dithering, which looks similar to Noise dithering, producing a random-looking pattern of dots.
Good if the dithering effect looks bad, but can produce severe “banding” of colours.
No Dither: No dithering is applied to the image. Dithering involves adding patterned or random dots to the image to make it appear to contain more colours than are actually in the palette, allowing you to use a smaller palette size. This is the drop-down box below the “colour reduction method” box. These days, this usually isn’t that important as most displays support at least 16 million colours. Web: Use if you need a complete Web-safe colour palette (Netscape 216-colour palette). Generally not quite as good as Selective. Adaptive: Creates the palette by taking an even sampling of colours from the image’s colour spectrum. Selective: Like Perceptual, but better for Web work. Perceptual: Creates the reduced colour palette favouring colours that the human eye is more sensitive to. This is the drop-down box below the Image Format box. Some of the important optimisation options are discussed below. You can then fine-tune the optimisation to suit your needs. If you’re making a GIF, start off with a preset such as GIF 32 Dithered, which should work well for most GIFs. For a full tutorial on choosing the right image format, see Understanding image formats. As a basic rule, use JPEG’s for photos and GIF’s for everything else. There are three basic image formats in the presets – GIF, JPEG and PNG. Click the Settings: drop-down list (just below the Cancel button) to bring up the list of presets: There are a lot of options in the Save for Web dialog, and the choice can be quite bewildering! Luckily, there are a list of presets you can choose from to make it easier. You can also use the Hand tool top-left to move the view around (if the image is bigger than the view), the Zoom tool to zoom in and out of the image, and the Eyedropper tool to select a colour from the image, to be used by various colour options on the right of the dialog. The four tabs above the image view pane on the left allow you to view the original image, the optimised version (this is the default), the original and optimised versions side-by-side (“2-Up”) and the original, optimised and two variations on the optimisation all together (“4-Up”). You can select the file format to produce (GIF, JPEG or PNG), what size palette you’d like to use, how to cut down the colours to fit the chosen palette size, how much to dither the optimised image (if at all) and how much to sacrifice image quality to produce a smaller file size. 
This is where you tell Photoshop how you’d like to optimise your image for the Web.
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