See the discussion in the Short Takes section of this site.

Recommended Reading:  Bill Kovach and Tom Rosenstiel, The Elements of Journalism:  What Newspeople Should Know and the Public Should Expect (New York: Crown Publishers, 2001); Gaye Tuchman, “Objectivity as Strategic Ritual:  An Examination of Newsmen’s Notions of Objectivity,” American Journal of Sociology 77 (Jan. 1972): 660-679.