What is sulfate? (magnesium sulfate/epsoms) dry skin..

I looked up sulfate and even after reading the definition I still dont understand. Is sulfate a form of sulfur?
Im trying to figure out why taking magnesium citrate makes my skin soft but taking epsoms/magnesium sulfate makes my skin dry. I dont get diarrhea with either so I do not percieve a loss of electrolytes. Does sulfate block any other minerals absorption? Anyone know, please educate me.