Excerpt from the Canon Boot Camp Reference Manual
Canon EOS Cameras – The 1D, 5D, 60D, 7D, T3i, and T2i are in the family of digital still cameras. Hundreds of Canon lenses are available for these Canon cameras.
EOS stands for Electro-Optical System. There are electrical contact points on these lenses so the camera’s computer can operate the lens f-stop and focus. Data about the camera settings also passes from the lens to the picture data.
The EF lenses have an “Electronic Focus” capability. Data from the sensor is transmitted to the lens to adjust focus until the sharpest picture is found. The EF lens is popular and other manufacturers make EF lenses.
Canon EF lenses are available all over the world. You can rent them easily and you’re never far away from a back-up lens if you need one.