Elementals have the ability to manipulate and withstand certain types of energy or substance.
Power levels vary, one fire elemental my be able to withstand and manipulate an inferno, another may be only able to light a match.
There are also cross elementals (those with more than one element) but these are extremely rare.
Types of elementals:
Some elementals are able to use another element to fuel their powers, for example Sanura is able to use light to fuel her dark energy elemental powers.