Web pages can contain many elements—HTML text, bitmap images, and vector graphics, to name a few. In Illustrator, you can use slices to define the boundaries of different web elements in your artwork. For example, if your artwork contains a bitmap image that needs to be optimized in JPEG format, while the rest of the image is better optimized as a GIF file, you can isolate the bitmap image using a slice. When you save the artwork as a web page using the Save For Web & Devices command, you can choose to save each slice as an independent file with its own format, settings, and color table.
Slices in an Illustrator document correspond to table cells in the resulting web page. By default, the slice area is exported as an image file that is enclosed in a table cell. If you want the table cell to contain HTML text and a background color instead of an image file, you can change the slice type to No Image. If you want to convert Illustrator text to HTML text, you can change the slice type to HTML Text.