An art critic is normally a person who have a speciality in giving reviews mainly of the types of fine art you will find on "display". Typically the art critic will go to an art exhibition where works of art are displayed in the traditional way in localities especially made for such exhibitions. A professional art critic could be expected to have great knowledge of both contemporary art and the history of art, and thus being able to make informed assessments of the art in question. Knowledge however provides no guarantee that a critic may be able to know if a piece of art, an exhibition or an artist will stand out in the history as "great".
Many now famous and celebrated artists were not really recognized by the contemporary art-critics, often because their art was in a style ahead of their time. Conversely, some critics have helped to explain and promote new art movements which is what Roger Fry achieved with the Post-Impressionist movement.