Name singular: Deity/God
Name plural: Deities/Gods
Lifespan: Immortal.
Realm: Deities originate from the Divide, the space between the realms. Due to their higher existence, they are typically imperceptible to other beings in the Divide.
Powers/abilities: Being literal gods, deities come with all the standard stuff — they are immortal and can only be killed with magic (typically very powerful magic, at that). They can easily will matter and energy into existence — therefore allowing them to conjure up any object. They can take on any form that they please, teleport, and easily move from one realm to another. From their home in the Divide, they can also "watch" other realms — keeping an eye on the goings-on of a particular world as if looking down from above.
While deities are of course very powerful, not all of them are completely omnipotent or omniscient. There exists a sort of hierarchy of gods (more on that later). Additionally, most gods have a certain 'specialty' (or set of specialties) in regards to what their powers pertain to. A god of love and peace would not have quite the same powers as a god of war and hatred. Typically, gods at the top of this hierarchy are closest to having 'infinite' power, as their rule over the most broad and all-encompassing themes, while gods relating to much more specific themes are generally less powerful and less influential.
Physical description: Deities have no concrete physical form. In their most "natural" state, they are intangible and ethereal — more akin to pure souls than anything else. They are capable of taking on any form that they wish — be that form an ethereal projection or a physical body — and the appearance of a deity's form at any given point in time often differs based on what realm they're in and what their intentions are.
Cultural description: The "society" of deities is a complicated one. They are ethereal beings by nature, and exist in a state that is hard to understand. Imagine a thousand conscious minds all melted down and mixed together into a slurry. Imagine that each mind can easily communicate with each other because of this closeness, but with each one maintaining its autonomy. Now imagine that each mind can "think up" a physical form for itself, or an environment to exist in, and that the minds can also interact with each other in that way. This is a general idea of how deities exist.
The problem with this kind of existence is: it can be very boring. This is why deities create worlds which can then act independently of themselves, and which the deities can enter and play around in. Sometimes a deity may want to simply take on a mundane physical form and live among the mortals for a while. Sometimes they might have something bigger and flashier in mind. It just depends on what they feel like doing that day.
The gods that have existed for the longest are the ones who have been around at the beginning of time. They created the universe and shaped everything in it. Of course, this is not to suggest that a more scientific understanding of the world's origins would be completely wrong. Instead, think of it this way: the gods are the ones who wrote the laws of physics, and who exist as a guiding hand among all the so-called "entropy" in the universe. Events can appear to be random, but actually consist of carefully-decided outcomes, all leading towards a certain goal. Life on earth (and elsewhere) may have evolved from the simplest of single-celled organisms, but with a team of creative minds guiding each genetic mutation along the way. Like, did you really think that platypuses exist just because? No, platypuses exist because someone thought it would be hilarious and decided it needed to happen, no matter how many weird evolutionary twists and turns it would take to get there.
Many gods, particularly the older ones, also have control over how various realms interact with one another. Afterlife realms are the greatest example of this, as there are gods who decide which afterlife specific beings will enter when they die, and generally maintain the goings-on of these realms.
There is a certain unspoken rule among deities, that each deity should be left to maintain whatever world(s) they created, and that other deities should not interfere in massive ways. It also helps that the deities who created most realms — especially the realms that get the most attention — are also typically older and more powerful. In other words, people you wouldn't want to piss off. This is what makes it so that any old deity can't completely destroy earth on a whim.
Deities can come into existence in one of two ways. The first is to be created by another god. This could mean being a "child" of that god, but such a process is a bit hard to explain, and doesn't typically follow the biological process of baby-making as we would understand it. A powerful-enough deity can also create a new deity out of essentially nothing, therefore not needing a second "parent".
Additionally, other races are capable of "ascending" to godhood and becoming deities — although, this typically does not happen without the individual in question being hand-picked by existing deities for such a purpose. There would have to be something special about them in order for an existing deity to think they were deserving of it.
Demi-gods — beings who were born of one deity and one mortal parent — can also exist. In fact, their existence is not that uncommon, given how easily bored deities can be and how often they tend to hang around with mortals. The abilities that a demi-god can possess vary widely based on the abilities of the deity parent and the race of the mortal parent. However, they are almost always limited in their powers, and therefore really aren't comparable with "pure" gods. However, they can ascend to full godhood if enough existing deities decide it should be so — which would give them the full range of deity powers, just like any other ascended being.
It was mentioned that there exists a "hierarchy" of gods, though this is more informal than it sounds. Essentially, some gods are more powerful than others, and it is the more powerful gods who are seen as authority figures, capable of keeping lesser deities in line. The gods who are more powerful are typically the ones who have existed since the beginning of time, or close to it. Younger deities, as well as deities who ascended into godhood out of mortality are generally lower on the hierarchy (depending on their age and how great their power is). Demigods are almost always at the bottom, usually not even being seen as "true gods" at all, unless they ascend fully into godhood.
Short description: Deities are the beings who create, maintain, and watch over all realms in existence. They are immortal, and their abilities can include almost anything. Their boredom often drives them to taking on physical form and interacting with mortal races.