Ask them, and they will give you different answers to this very same question. Well, let little old me sit down and tell you just how it is.
An Otherkin is a member of the Otherkin sub community.
Well, it's just that not everyone who shares the characteristics of what makes an Otherkin Otherkin is part of the sub community - nor do they consider themselves Otherkin, you see? The Otherkin are tribal that way. So, let me explain a little further.
Members of the Otherkin sub community do not consider themselves human for the most part. (There are some that do.) Their reasons are varied, but generally run along the lines of physical nonconformity with the human race, past life disassociation, spiritual separation (i.e. they could be a walk-in), and/or just feeling like they don't belong.
Any further questions? No? Good.