What's the difference between snow cover and fractional snow cover?
Fractional Snow Cover (FSC) is a property of snow cover.  The fractional snow cover property is used in a variety of contexts, most common of which is meteorological models.  Snow cover algorithms applied to visible imagery remote sensing data are used to derive fractional snow cover for each pixel.  This information is typically re-gridded and time-averaged for use by external users.  

This GOES site explains:


  Most satellite-based snow cover products provide a "binary" map, whereby each pixel is classified as either "snow" or "not snow". In contrast, the FSC algorithm is an optimized spectral mixture analysis that uses the broad spectrum from ABI to determine the fractional cover of snow, vegetation, and soils.


More information about snow cover products from GOES and MODIS can be seen here:

http://www.goes-r.gov/products/baseline-snow-cover.html

https://modis.gsfc.nasa.gov/data/dataprod/mod10.php