As you can see, several different levels of compression may be tried to get a good image, yet keep the size small. If an image takes too long to load, your visitor may leave the site. The general rule of thumb is keep the page and all associated single-use graphics under 40K in size. Transparency is a color, or mask, that the program saving the image makes clear in the final version. Color table size specifies how many unique colors are used in the compressed image.
Fewer colors means a smaller file size.
Dither uses a few colors to simulate many by using them close together. Transparency is an option that you didn't have when working with JPEGs. All of the GIF demo images were saved with no dither . When dither is enabled, colors will be chosen for the table that can be put together to simulate another color. Dither , as you can see if you look closely, uses fewer colors, as shown in the right-hand image.