Mizunoman's spot on with the technical differences. As far as choice of material for the clubhead is concerned there's now such a wide variety (carbon steels, stainless steels, copper, beryllium copper, titanium and all sort of other combinations) each with varying characteristics. The behaviour of a gold ball of each material is governed by the mechanical properties of the material (elastic modulus, hardness etc.) but leave all that to the club manufacturer's R&D and marketing departments (I was trained as a metallurgist and there's two parts science one part mumbo-jumbo in this). The best advice is simply to try them all and see which one gives you the best control and feel regardless of head and shaft combo.