NIKON AF-S FIT
Focusing is driven by a "Silent Wave" motor in the lens instead of the focus drive motor in the camera. AF-S lenses focus faster than standard AF-Nikkors and almost completely silently.
CANON EF-S FIT (Cropped Sensor)
The Canon EF-S mount only fits digital SLRs with cropped sensors. The S stands for short back focus and means it's compatible with SLRs featuring smaller mirror boxes.