EF means that it is an electronic focus lens, basically every lens made since the late 80s. Also called auto focus. EF-S are crop sensor camera lenses. They will only fit on APS-C cameras, which as of 2009 is everything but the 1D and 5D. EF lenses will work on APS-C bodies as well as full frame and film, but cost more in most cases. L series lenses are top of the line lenses. If you buy an L lens you know you are getting the best optical quality available. You will also pay dearly for it.