Something that's expressionistic uses emotions rather than realism to express an artistic idea. An expressionistic painting gives you the general sense of a tree, rather than a photographic duplication of a tree.
Expressionistic art, which flourished at the start of the 20th century, used distortion and exaggeration to get feelings and ideas across. Many painters we admire today, including Matisse, Monet, and Van Gogh, all used expressionistic techniques in their work. Other forms of art, including poetry, prose, and dance, can also be done in an expressionistic style. The root word is expression, used in the art world to mean "way of expressing."