Every EF lens is identified by a series of codes which identify important information and features.
For example, the EF-S 15-85mm f/3.5-5.6 IS USM is a lens with an EF-S mount, a zoom focal length range from 15mm to 85mm, a maximum aperture that varies from f/3.5 to f/5.6 (depending on the focal length set), an ultrasonic motor (USM) for focusing and built-in image stabilization (IS). What follows is a more in-depth description.
EOS cameras use one of three different lens mounts. Every film and digital EOS camera accepts every EF lens (the EOS M requires Mount Adaptor EF-EOS M).The EF (Electro-Focus) lens mount was introduced in 1987 and was the first to use electronics for all communication between the camera body and lens. The focusing motor is inside the lens (rather than the camera body) which means it can match the particular characteristics of that lens.
Canon lens mounts. From left to right: EF mount, EF-S mount and EOS M mount
EF-S lenses tend to be smaller and lighter than their EF equivalents. EF-S lenses cannot be attached to cameras that only have the red mounting dot (full-frame and film models) - they only fit cameras with the white mounting square.
The new EF-M mount is smaller than the existing EF/EF-S mounts. The size of the mount means that the EOS-M can offer the quality of a digital SLR in a compact body. It has a white circle for the mounting index. Only EF-M lenses can be fitted direct to an EOS-M but if you attach the Mount Adapter EF-EOS M, any EF or EF-S lens can be used.