What is the difference between normal salt and salt of guérande?
Salt is salt is salt.

That being said, most regular salt comes from salt mining, which tend to be neutral as there is no (or lot less) additional minerals (or other stuff) in the salt that will change the taste.

Sel the Guérande is a sea salt, it comes from evaporation of sea water, since the sea water contains a lot more minerals and "stuff", like algae.

Depending on the production method, the resulting salt can have different shapes and texture (e.g. guerande is flaky, Maldon is like a "pyramid"). 

The resulting taste will be subtly different, and will have a different feeling in the mouth when used as a finishing touch on a dish.