Why does ethnicity play a role in the USA in 2013? Is someone born here to parents who came from Asia considered "Asian"? Because he/she looks Asian? And what i

Why does ethnicity play a role in the USA in 2013? Is someone born here to parents who came from Asia considered "Asian"? Because he/she looks Asian? And what if he/she is 4th generation in the USA and looks Asian, is that considered Asian? This is baffling.