EF lenses (Electro Focus) automatically focus electronically using a motor that is built into the lens body. They focus very fast and quietly. Communication between the EF lens and the digital SLR camera body takes place via electronic contacts on the bayonet (mount).
EF-S lenses (Electro Focus Short Back Focus) are designed for digital SLR cameras with smaller APS-C sensors. EF-S lenses are smaller and lighter design, and sit closer to the sensor.
