Expressionism is, generally speaking, a tendency in any art form (painting, literature, film,architecture and so on) to distort reality for emotional effect. Additionally, the term often implies emotional angst - the number of cheerful expressionist works is relatively small.
In this general sense, painters such as Mathias Grünewald and El Greco can be called expressionist, though in practice, the term is applied mainly to 20th century works.
Some of the movement's leading painters in the early 20th century were:
Blaue Reiter and Die Brücke. Later in the 20th century, the movement influenced a large number of other artists, including the so-called abstract expressionists.
Expressionism is also found in other art forms - the novels of Franz Kafka are often described as expressionist, for example, and there was a concentrated Expressionist movement in early 20th century German theatre centred around Georg Kaiser and Ernst Toller. In music, Arnold Schoenberg and Alban Berg both wrote pieces described as expressionist (Schoenberg also made expressionist paintings).
In architecture, the work of Eric Mendelsohn comes under this category. An important building by him under this style is the Einstein Tower in Potsdam, Germany. There is an organic quality to buildings using this approach.
There was also an expressionist movement in film: see expressionism (film).