The difference between a French Whisk or a French Whip and a Piano Whisk is that the French Whisk and French Whip has thicker wires and is used for whipping or whisking thicker ingredients.
The Piano Whisk has thinner wires and is used for whisking and whipping up thinner consistency ingredients.
In simple terms the difference between a French Whisk and a Piano Whisk is one is larger than the other and the French Whisk is larger and contains larger wires than the Piano Whisk.
However sometimes the Piano Whisk may be larger in size but have smaller wires than the French Whisk.
French Whisks are meant for thicker ingredients while the Piano Whisk is meant for thinner consistency ingredients.
However you can use the French Whisk for whisking thinner ingredients if you want too.