Scholars have spent much time studying the elements that druids manipulate, and there are two reigning schools of thought regarding the use of elemental magic.
Druids believe that the power to manipulate the elements is a gift given to them by the forces of nature, and that elemental magic should be approached in a spiritual way: namely, that the magic should be used to maintain and defend the balance of nature.
Elementalists believe that the power to manipulate the elements comes from within the caster, and that rather than being given a gift of nature, they are instead honing their will to be able to exert it over nature and bend it to their whims.
In short, Druids believe that their power is external, while Elementalists believe that their power is internal. Both schools of thought produce powerful Elemental Mages.