The Elemental Planes
The Elemental Planes are realms of pure elemental chaos. From the swamps of the plane of Clay to the endless storms of the plane of Lightning, every piece of these planes is something unique. All of this chaos turns into an energy that flows into the material plane to shape it. A byproduct of this transfer of energy is what most people understand as Mana. Mana is a system of this trade, tied to both the Material and Elemental Planes. Everything in the world is tied to it, living or not.
Imagine the Elemental Planes as a cloth, a weave that lies over the Material Plane. In some places, the weave of that cloth is thinner, more elemental chaos can leak through, while in other areas the weave is thicker, and less gets through. In some areas, where the cloth is thinnest, the cloth will touch the Material Plane, creating a pathway through to the Elemental Planes, commonly referred to as a Gate.
The Gates
Millenia ago, the weave of the Elemental Planes was ever shifting, as were the Gates. The Gates never stayed in the same place for long. This created an atmosphere much like our Earth. Some slight extremes, but nothing unhealthy. Things were free-form. However, ancient peoples realized they could manipulate the Mana in the world around them. As more people tried to pull at the Mana around them, the weave of the Planes got thinner and thinner, and was pulled closer to the Material Plane. Eventually, these peoples realized they could use their mana at the thinnest points of this weave, and permanently tie a Gate to that place in the world. Think of it as taking the cloth that is the Elemental Planes, and sticking a thumbtack straight through it.
As you can imagine, the world did not like this. The Gates were meant to be ever shifting. Them staying in one place meant that the majority of elemental chaos from each individual plane came out in those regions the Gates were tied to. This caused the world to go into a near apocalyptic state, with the amount of terrible natural disasters going on in specific regions. However, over the millenia, the mana the Gates themselves exude allowed them to continue to stay tied, and stabilize themselves somewhat.
All elements have their Gates, except for the Planes of Light and Dark. They are planes separate from the Elemental Planes, and while they both do still feed into the Material Plane and people can have affinities for them, nobody is purely either. They act more as modifiers for the elements they are mixed with, rather than pure elements on their own.
Gatekeepers
Nowadays, there are people by every Gate, tasked with its protection. These people are known as Gatekeepers. There is one Gatekeeper per gate, though they tend to have what seems to be a small army of mages working underneath them. These Gatekeepers function in a few different ways, depending on their Gate. For more unstable elements, its Gatekeeper mainly focuses their efforts towards pouring mana into the Gate to keep it stable. For more stable elements, its Gatekeeper is more focused on protecting their Gate from outside sources.