there's stuff on it in the revise a month book. these are my definitions from it:
Art Critic: The critic could be thought of as a navigator who offers a way of reading a map about the visual arts. The objective of the critic is to clarify and assess: the qualities of an artwork, how an artist develops and produces art and, how the audience may view, understand and appreciate the artwork.
Art Historian: Map out the significance of artworks and artists in relation to their contexts: that is, the periods of time and the places in which they worked. Historians are interested in tracing how artists and artworks gain status and how audiences relate to the artworks.