The Din are a category of humanoids characterized by their possession of a Focus. A Focus is a sort of practice, aversion, taboo or belief that grants the Din immortality so long as it is continuously observed, as well as certain other powers or abilities, based on the sub-race of Din.
The Din themselves often quarrel over what precisely constitutes a Din. The Ohanedin, for example, maintain that Humans are also Din (they call them the Hostoadin) because of their ability to Ascend, which they consider a form of Focus. The Burdinadin, by contrast, do not believe that the Zells or the Ohanedin are Din at all, because both receive their immortality from higher powers, rather than from within. The Orredin believe that the Goblins are a degenerate and fallen form of Din that has forgotten its Focus (and thus, justify by precedent that the Orredin themselves are an ascended, superior race, while the rest are merely average).
In terms of appearance, the Din as a whole are generally similar to humans, but are usually marked by slightly tapered ears (though this varies between the modest and delicate Orredin, and the grand, some would say obnoxiously large Zells). Humans (for those Din who hold that the Humans can be counted among them) are simply a logical continuation down from the Orredin.