There are eight beings that embody the elements of the world: Earth, Wind, Fire/Heat, Water, Electricity, Ice/Cold, Light, and Dark.
There is one elemental to each element at a time and no more than that. These entities could be almost anyone in existence.
Two of the elements are each opposites and in general, are weak against each other (usually).
Earth & Wind
Fire & Ice
Electricity & Water
Light & Dark
Some of the Elementals aren't even aware of their status as an Elemental.
Generally, elementals don't get along with their opposites. They may be able to have a civil conversation with their opposite, but expect the two to be tense. All elementals can recognize each other by instinct once they are aware of their own status as an Elemental (and with opposites, this only serves to intensify feelings of unease and such). This recognition may be blurred or made uncertain if the one being seen isn't aware of their status (though the observing Elemental may still correctly ID who they're looking at based on abilties or appearance).
If one is killed at any point, they will be reincarnated at some point in time. It could be anytime from a few days to several decades before they return, but so long as the opposite elemental exists, they will return at some time. However, they will be an entirely new character then, they may have a different name (this, strangely has never been the case with Dark), and under normal circumstances, they won't have any memory of their previous existance (though they may hear about it).
Characters listed here that aren't actually Elementals show potential to be an Elemental.
Elemental Embodiment, Feed it with Fire/water/earth/etc. (elementals typically can't really be harmed by their own element, there are exceptions though), Reincarnation, Apocalypse How (a slow Class 6 if all elementals are permanently dead)