Question

What is the primary difference between a citizen and a non-citizen?

Answer 1

A citizen can elect and be elected, a non-citizen doesn't have this right.

Answer 2

I fail to see the biological relevance, also I think the answer my vary from country to country. So I give an example from my homeland.
A non-citizen does not have civil rights and the civil rights include especially the right to vote or stand for election, the right to stay in the country and protection from extradition to other countries and the possibility of being hired as a state official.