You may already be familiar with the technique of changing the background image in order to create purely CSS rollovers (i.e., ditch that pesky onmouseover in favor of :hover). The concept of "CSS sprites" takes it a bit further.

The idea? Consolidate all your images into one big image, and just shuffle the background position depending on what you want the user to see.

This article at informit is an excellent introduction to CSS sprites. It breaks down the process step-by-step, providing examples at every point along the way. And check out a great example by Dave Shea that really drives home the extent to which this technique could be utilized.

On large sites, every little server call matters. This is one way to minimize server calls while maintaining separation of content from presentation, accessibility, and graceful degradation.