The Canon EF-S lens mount is derived from the EF lens mount and was created in 2003 for Canon digital single-lens reflex cameras with APS-C sized image sensors.
Lenses with EF mount can be used on APS-C sensor cameras while lenses with EF-S mount cannot be used on full frame (35 mm) cameras. Flange distance and diameter mount in EF-S are the same as in the EF mount but in EF-S mount the back of the lens is closer to the sensor, allowing lenses to be smaller and lighter, though of course with smaller sensor - around 1.6 crop factor with respect to full frame.
The "S" in EF-S can be first letter of "Small image circle" or "Short back focus".
|Sensor format||Sensor crop factor|