An Elementalist, quite simply, is to a warlock what a wizard is to a psi-mystic. Whereas the Warlock gains limited insight based on his link to an elemental force, an Elementalist studies the full range of elemental magic, using his knowledge to manipulate the world around him.
More specialized than a Wizard, an Elementalist has trouble casting many of the spells that a Wizard takes for granted... in fact, of the General Spell Magic lists of a Wizard, only a few are commonly available to the Elementalist. On the other hand, the full range of elemental spells are available to him, making him (or her, though men seem to out number women in this discipline by about 3:2) a very powerful sorcerer in his own right. It is rumored that the great Tezaun was an Elementalist, but whether or not this is true or simply an attempt by modern elementalists to borrow mystique from this near-mythical figure is unknown.
Historically, these magicians were one of the few disciplines to survive the Millenium of Purification, though not for lack of trying. Many warlocks see them as being perversions of "true" magic and enslavers of their elemental brothers (which some are), and they were hunted mercilessly during that time. The Warlock ability to recognize other Warlocks played a deadly role in this conflict, since they could quickly tell a "False Warlock" from a true one, and band together to obliterate him. Many Elementalists, as well as Wizards who became too well-known for their use of Elemental magic died at the hands of these "crusaders". Elementalists, survived, however, in ones and twos, rediscovering their magic over the long centuries, until they have become a small, but important, part of the Palladium World's magical landscape.