Elementals are spirits of the raw stuff of creation: earth, air, fire, and water. Wild and dangerous as the elements that birthed them, they flow freely throughout their planes, and when they want to interact with their environment, they can assume material form by wrapping a portion of elemental substance around themselves. They exist to act as incarnations of the elements, but they're also free-willed creatures with societies, leaders, fears, and aspirations. They tend to be much more social and intelligent on their own plane than a spellcaster who's only summoned them would be led to expect. The vast majority are neutral in alignment, though some few have been convinced to serve good, evil, chaos, or law.
Similar creatures include paraelementals and quasielementals; these are spirits of the same nature, although they represent more complex substances: ash, dust, salt, vacuum, radiance, mineral, mist, lightning, magma, ooze, ice, and smoke.
Exactly how elementals come into being and how their hierarchy is decided is unclear. However, I will enumerate their levels of relative power, in order of least to greatest: