Gothic is the term used to describe someone who dresses in a dark fashion. People in this culture additionally wear dark make up and usually have geometric hair styles. The style was brought to the main stream by such artists as Marylin Manson.
A gothic person could refer to many things. There are East Germanic tribes which are known as goths. Gothic could also refer to late middle age art. For more information see here: http://en.wikipedia.org/wiki/Gothic