Realism - The attempt in literature and art to represent life as it really is, without sentimentalizing or idealizing it.  Realistic writing often depicts the everyday life and speech of ordinary people. 

Naturalism - An extreme form of realism.  Naturalistic writers usually depict the dark side of life and show characters who are severely, if not hopelessly, limited by their environment or heredity (especially Crane, Dreiser and Norris). 
John Steinbeck sometimes used naturalistic techniques.  His novel, The Grapes of Wrath, shows a poor migrant family ruthlessly crushed by natural and social forces, over which they have no control.