The term "Expressionism" in art does not refer to a movement or school, but a work created to distort reality for an emotional effect. A well known example of an Expressionist painting is "The Scream" by Edvard Munch.
It usually applies to nineteenth and twentieth century art, including film, theatre, architecture, music, literature and painting. It evolved from a period in German art that challenged academic standards and expressed pure and intense emotion. It is now applied to works that see the creator or artist abandoning procedure and trained technique in service to the inner emotions of the artist.
There has always been an element of the emotional in almost every conceivable style of art, but where Expressionism is concerned the production of art is focused on communicating strictly through emotion and not a particular use of light, paint, composition, etc. It frequently appears in conjunction with any social upheaval as well as any internal or societal chaos. Normally the works are not found aesthetically pleasing or impressive, but are able to produce a strong emotional response from the viewer.
It is usually seen as the opposite of Impressionism because of its goal to reject purely surface concepts and external stimuli and instead turn inward and express the feelings of the artist.
There are a few groups that casually organized themselves as Expressionists, and they include the Der Blaue Reiter, Die Brucke, and the Bauhaus School. Additionally, well known Expressionist painters include Wassily Kandinsky, Amadeo Modigliani, Thomas Hart Benton, Marc Chagall, Milton Avery, Willem de Kooning, Walt Kuhn, George Grosz, Franz Marc, and Vincent Van Gogh, among dozens of others.