The Canon EF (Electro-Focus) is Canon’s lens mount for the Canon EOS (electro-optical system) system. The EF mount was introduced in 1987 for Canon film SLRs, and is still in use today. Brand new lenses have full compatibility with film SLRs from the 80s. The EF mount is fully electronic, as opposed to the mechanical connections of Canon's previous FD mount. Instead of incorporating autofocus into their existing mount, like Nikon and Pentax did, Canon opted to start from scratch. They abandoned the FD mount and went with lenses with built-in focus motors. It was a bold move, but one that paid off for them in the end.
One of the main advantages of the EF mount compared to Nikon F, Pentax K and Sony A is the shorter flange distance. The EF mount has less distance between the back of the lens to the image plane, allowing for the easy use of lens adapters. A shorter flange distance also allows for more effective wide-angle lenses at lower prices.
Because of the EF mount's success, cameras, lenses and accessories are widely available for approachable prices.