This is a very goog question. I use the concept of human nature a lot in my posts and I just realized that I don't even have a basis way of describing what I think it is, and after thinking about it I am still lost :)

Although I have no idea how to describe it, I think I understand part of it. As an example, I think that we have a tendency to aspire for the better, we also use a combination of feelings and logic in relating to things, but this is still very vague :)

Maybe human nature is what rests between biological imparatives and cultural norms.