Expressionism is the tendency of an artist to distort reality for an emotional effect
it is a subjective art form.
Expressionism is exhibited in many art forms, including
painting
literature
theatre
film
architecture
music
The term is applied mainly to 20th century works.
Originated in Germany
Started after the first World War
Took inspiration from
Symbolism
Fauvism
Cubism
Abstract Expressionism
Developed in the united states after the end of world war 2
New York Figurative Expression 
of the fifties represented American figurative artists such as:
Willem de Kooning
Grace Hartigan
Lester Johnson
Alex Katz
George McNeil
Jan Muller
Jackson Pollock
Fairfield Porter
Larry Rivers
Bob Thompson
Lyrical Expressionism Tachisme
of the 1940s and 1950s in Europe represented by artists such as
Georges Mathieu
Hans Hartung
Nicolas de Staël and others
North American Lyrical Expression
Started in the late 1960s and the 1970s. Characterized by the work of
Dan Christensen
Peter Young
Ronnie Landfield
Ronald Davis
Walter Darby Bannard
Charles Arnoldi,
Pat Lipsky and many others.
Neo-expressionism
An international revival movement beginning in the late 1970s and centered around artists across the world:
Germany:
Anselm Kiefer
Georg Baselitz and others;
USA
Jean-Michel Basquiat
Eric Fischl
David Salle
Julian Schnabel
France
Rémi Blanchard
Hervé Di Rosa and others
Italy
Francesco Clemente
Sandro Chia
Enzo Cucchi
England
David Hockney
Frank Auerbach
Leon Kossoff
Christopher Le Brun