CSS sprites are a way to reduce the number of HTTP requests made for image resources referenced by your site.

Images are combined into one larger image at defined X and Y coorindates. Having assigned this generated image to relevant page elements the background-position CSS property can then be used to shift the visible area to the required component image.

This technique can be very effective for improving site performance, particularly in situations where many small images, such as menu icons, are used.

http://www.de.spritegen.website-performance.org