A photocopier can be used to reduce images, enlarge images (cropping), combine images and copy actual objects and materials. You can also obtain interesting effects by adjusting the contrast if this is available.
Before any discussion of this fascinating medium begins, however, it is necessary, though not entirely possible, to define what is generally understood to be photocopy art, or as it is usually referred to, Copy Art. Copy art "is anything that has been created, transformed or enhanced through the use of the copy machine". http://collections.ic.gc.ca/waic/dissertation/daliss_copy.htm