As you know many links in a website makes it slow loading. CSS sprites allows you to greatly increase your websites speed by using single image files that contain multiple graphics. That means you can load more graphics with a single server request, juts by combining the number of images and using them in sections where needed. This can greatly reduce loading time and the total size of a web page, which will eventually help people navigate your pages faster. The theory behind CSS Sprites is just get the image once, shift it around and only display parts of it, saves the overhead of having to fetch multiple images. Nowadays, when websites compete for the best user interface, load time and the size of a web site are some of the biggest factors to consider.
Most professional web designers use this technique in website development. Each one of those images is a separate HTTP-Request, and the more of those, the less efficient your page is in terms of loading time. This technology has saved companies like apple and yahoo thousands of dollars a month on server requests. The difference is that instead of just two or three images being combined into one, you can combine an unlimited number of images into one. Major websites like Facebook, Yahoo, Google, Myspace, eBay and more use CSS Sprites.
For your simple understanding , if you are wondering how could you combine multiple images, then here’s the CSS Sprites used by YouTube & Facebook.