Abstract Expressionism
Abstract expressionism, also known as the New York School, was a movement that began in New York and the surrounding areas in the 1940s. Large-scale works using big canvases, spontaneity, improvisation, and the importance of the process of creating the artwork characterize it.
The most mature and enduring examples of the movement have two focuses: dynamic and energetic gestures that are often coincidental, and reflective open fields of color. Though the two styles are very different, the main concept behind them was the same, which was to create art that was free from premeditation. Above all, the immediacy of expression was the most important.
The first style was the more dynamic of the two. Artists of this style were Jackson Pollock, Lee Krasner, and Franz Kline.
The second style, widely acknowledge as the more “cerebral” of the two, was characterized by large color-dominated fields that were meant to be reflective. Most of the works in this style, especially Mark Rothko’s, were large canvases that were “big enough to envelop.” Another artist of this style is Barnett Newman.
Source: http://www.metmuseum.org/toah/hd/abex/hd_abex.htm