Abstract Expressionism was big in the 1940’s. Subject matter was usually non-representational as artists created with the intent to show an emotion. Not all of it is any of these though. Abstract Expressionism was as much about how artists applied paint as what the finished result was. They painted quickly in an effort to release their unconscious minds. A lot of it looks like it relied on chance or accident.

Sources: http://www.artlex.com/ArtLex/a/abstractexpr.html