Diane Arbus was a seminal figure in documentary photography. Her subjects were often members of society's fringe - an odd, discomforting, and new topic for most viewers in the 1950s and 60s. She violated all previously implied "polite distance" rules between herself and those she shot, getting close as she felt warranted. She also had an unsurpassed ability to inject a certain amount of psychology into any given picture, as if compelling the viewer to look long and deeply.
Arbus is best known today for the "freak" photos that were so sensational 40-odd years ago, and for becoming something of a cult figure by dying young. It is difficult to locate more than a handful of Arbus' works on the Internet, due to her closely guarded estate. For extended viewing, one's best bet is either purchasing or borrowing one of the image-intensive books on the artist authorized by the Arbus Estate.