Expressionism in literature arose as a reaction against materialism, complacent bourgeois prosperity, rapid mechanization and urbanization, and the domination of the family within in pre-World War I European society. It was the dominant literary movement in Germany during and immediately after World War I. In forging a drama of social protest, Expressionist writers aimed to convey their ideas through a new style. Their concern was with general truths rather than with particular situations.

Realism in literature, means the theory or practice of fidelity to nature or to real life and to accurate representation without idealization of everyday life. The 18th-century works of Daniel Defoe, Henry Fielding, and Tobias Smollett are among the earliest examples of realism in English literature. It was consciously adopted as an aesthetic program in France in the mid-19th century, when interest arose in recording previously ignored aspects of contemporary life and society; Gustave Flaubert's Madame Bovary (1857) established the movement in European literature. The realist emphasis on detachment and objectivity, along with lucid but restrained social criticism, became integral to the novel in the late 19th century. The word has also been used critically to denote excessive minuteness of detail or preoccupation with trivial, sordid, or squalid subjects.

From what I've read (don't remember where!!) Realism was more of a 19th centuary concept that talked about the real life in America during that time. After the first world war, the writers were influenced by this style and went on to write about life in a realistic and unbiased way which we call modernism. Expressionism was a movement based heavily in Germany and was adopted by poets of the time like Gottfried Benn. It refers to the world in a subjective way to often to distort it to provoke emotional responses, moods and ideas. It was used to describe 'being alive' as more of an emotional experience rather than a physical one. more info http://en.wikipedia.org/wiki/Expressionism#Literature