When you slice an image, you divide it into several smaller images that you can save in different formats or at different levels of optimization. Because these optimized images can take less time to download than one large image, your Web pages may load more quickly.
The larger graphics programs have a slicing tool for this purpose. When you finish you can save the sliced up graphic for the web and your program will output it in a table format (the old constrained way of doing it). Now you slice out what you need and by knowing "css" code, you do your coding. Try this post, it's no 1,2,3 but may help understanding it
questions relating templates and how to program them before i venture into the known