The EF-S lenses are the "new" mount developed for the digital rebel series, which includes your XS, and it has allowed for the development of some less-expensive lenses for the amateur photographer. The EF mount is the only mount option on the professional end cameras, so the professional lenses only come in EF mounts. If you think you'll ever upgrade your camera body, you should probably only buy additional lenses with an EF mount, to make sure they'll be compatible later on.