Deities, more commonly referred to as gods, are beings of immense magical power who operate on a level far above most magicians and magical creatures. The Old gods are the primordial deities that responsible for the creation of the universe and its inhabitants itself, as well as the 'parents' of the modern deities from mythology.
Deities come in many forms and are as diverse as the humans who worship them, but can generally be characterized by their extreme durability, intelligence, nigh-immortality, and advanced magical ability. There are various kinds of gods and goddesses with different origins, though it can be inferred that their depictions in mythology is an embellishment of their true selves.
The deities that have been shown all have a megalomaniac view of themselves. They all treat humans as though they are disposable and that any interaction they have with them should be considered a blessing, stating that the mere act of sharing their seed is to be revered as a gift. Although, due to their virtual immortality and great talent, is it not much of a surprise.
Though unconfirmed, it would appear as though gods have a specific form as with Ember and Umber that can be destroyed. Reynard, on the other hand, seemed to be able to enter the body of another and work through them in a manner akin to possession, white only briefly showing his true beastial self.
It would seem that having the seed of a god inside of you gives you a lot more power than you can usually muster. This allows you to touch things that only a master magician can touch or cast spells of devastating magnitude. The effects of the seed wear off over time though.
Demigods are the children of a deity and a human. Due to their heritage, they hold an incredible amount of magical power. Demigods who are unaware of their true status might use their innate abilities subconsciously.
In the books, a human can become a demigod, or even 3/4 of a god.