A historian is one who studies history. Historians have existed as long as human history itself. Throughout the ages manuscripts and artwork can be found that relate to historical events, places and people.
However, a modern historian must always make sure that anything from a period historian is actually accurate. Historians all through antiquity have been known to write about things as if they were there, but in fact they may be writing about it 100 years later (cf. writing on the Crusades etc.).