Common Elementals

The common elementals are fire, earth, water, and air.  Most elementals are roughly humanoid.  Small elementals are roughly gnome sized, medium elementals are roughly human sized, and large elementals are roughly ogre sized.
For obvious reasons, no mortal has managed to enter the The Elemental Plane and survived the trip, so no one is sure what form elementals take in their home plane, but elementals in the material plane are well documented.
Elementals can be deliberately summoned by mortal magic users, or they can appear as "wild" or "feral" elementals.
If enough magical energy pools in one place in the material plane, it often the case that a feral elemental will appear. Feral elementals appear periodically around natural fonts or in areas where many magic spells were cast in a relatively short period of time.
Wild elementals often but not always act in a manner conducive to the emotional resonance of the magic that summoned them. If mystic energy is the result of a magical battle, the wild elementals spawned in the area are probably going to be belligerent and violent. If an elemental appears in an area where magic was used to aid in construction, the elementals accidentally summoned by them might try to stack rocks or otherwise build something, even if the something makes no sense or has no obvious use to mortal observers.
Summoned elementals essentially use the same mystic energy that summons "feral" elementals but they do it in a controlled fashioned.  This is also tied to magic to control the elemental themselves.  While elementals can break free of mystic control, they see to have less resistance to magical control than mortals, spirits, Fae, or even beasts.

Geographic Distribution

• the Elemental Plane