From what little I know...

Expressionism was initially a visual art movement in Germany pioneered by the groups Blaue Reiter and Die Brucke - look at paintings by people like Oscar Kokoschka or Wassily Kandinsky (himself a Russian) for examples. The term "Expressionism" was adopted to demonstrate the counterpoint to the French "Impressionist" movement. While Monet etc. painted landscapes, Expressionists believed that art should show the inner "Geist" of the artist and so concentrated on portraits of people in despair etc. Munch's The Scream is probably the most famous Expressionist painting.

This ethos was adopted later by composers (Schoenberg, Strauss etc.) and cineastes like Murnau and, to some extent, Fritz Lang.