The Gods are, well, Gods! They were the first to exist and rule pretty much everything. Sometimes they are more involved with helping start wars, calm them down, natural disasters or good years of harvest. Other times they sit back and watch the chaos or tranquility ensue. It really depends on their mood.
All three are equal in power and status, though they sometimes give one another specific roles if they feel like it. Tohma is the God the humans fear the most, though. While he can be the most benevolent, he can also be the most tricky.
The elements are lesser Gods, or more appropriately, Demi-Gods and Nymph-like creatures. They were created by the Gods to control the four main elements of nature; Earth, Wind, Fire, and Water, because it's just annoying to have to micromanage all of that! The Gods still have supreme power and can intervene in the doings of the Elements any time they wish, though they generally let them do their jobs. As for what they look like, they look somewhat humanoid but also hold aspects of their element or creatures that would exist within their element as well. Definitely recognizable as otherworldy.
These are also special creatures and not allowed to crossbreed, to preserve their specialness. However, this doesn't always stop them. The Elements can assume a completely human form if they so please, but only for a limited amount of time. At that time, they must return to their element to replenish strength. Elements can also transform into their given element at any time of need, though generally stay cloes to their element of origin. (EX: Mika, the Earth Element, would tend to stay near forests and plant life. Eiri would follow fire where it goes, Tatsuha would stay near rivers, streams, lakes, and the sea. And Ayaka...well, everywhere, if you think about it. Air is everywhere!)
As for the Gods? Well, they can do whatever they want with the Elements, basically. Because they're Gods.
These vary in size, shape, and power. Not Gods, but still mystical and sprite-like beings, they travel the land in various elements. Some even assist specific Elements in their duties as helpful faeries and the like. Gods can also turn humans into creatures if they felt like it, perhaps to fill a certain purpose or to assist an Element. They still rule supreme over all beings, including these. Strengths and weaknesses vary with the creature. Depending on what creatures sign up, we'll list them here as we accumulate them ^_^
The humans account for the bulk of this world's population. They were created by the gods, but no one is sure why. They go about their daily lives as most do. They fight with each other, build homes and families, hold jobs, beg and rule kingdoms - pretty much everything. They need to keep their eyes open for various things the gods may throw at them, though! However, if a God or Element takes a certain liking to them, the Gods may transform a human into a creature if they so felt like it.
Basically with humans, it's a free-for-all!