An art critic is someone who gives a critique and analyses art based on popular culture or quality where as the historian is more about the history and cultural context of the piece of art... For example the scream by Edvard Munch is visibly attractive and the critic may talk about the appearance and texture and ability of the artist and colour... But when the historian looks at it he may look for cultural relevance to the time it was painting and about the styles place in the era that the painting was painted :)