CSS sprites are simply a method used where you get all your small images used for a website and make 1 image out of them, then use CSS to select certain parts to display in the correct place. The idea being that all the background images, symbols ect can be downloaded as one file reducing number of requests and files used to display one page. If done correctly this will lower web page load times and actually help your SEO efforts. A good explaination can be found here - http://www.seoconsult.com/seoblog/page-load-and-seo/css-sprites-for-search-engine-optimisation.html