What is the difference between a slow sync and fill flash?

Slow sync uses a slow shutter speed in conjunction with the flash and is designed for shooting scenes in low light where the light from the flash cannot illuminate the background of the picture . The slower shutter speed allows ambient light to brighten the background.

A fill flash is used to brighten shadow areas in a picture caused by bright light in the background of the subject. This is useful on sunny days where the faces of your subject are shadowed due to harsh sunlight in the background.