French differenatiates between voiced and unvoiced sounds. The sounds concerned are s and sh.
What actually is the difference? The difference is in the use of your vocal chords. They vibrate in the unvoiced version. Actually English has the same. Voiced s are in the words crazy, these. Unvoiced s are in words like sun, fuss, Swiss.
Graphemes for voiced s are: s, z, x, zz
Also in this case there are word couples that only differ in the way the s is pronounced.