Abstract expressionism was a little earlier that Pop art. Abstract expressionism was defined by two things: 1, there is no representation of any object, person or place. Paint is paint, paint is not a person. For this reason, Abstract expressionists (2) also believed that when you use a medium, yous should create art that can only be done in that medium (i.e. pollocks dripping paint). The final image was up to the viewer to inturpret, not the artist.
Pop art was more about consumerism, and mass production. It was a reaction to the Abstract expressionists. Instead of an ambiguos image, pop artists hit you over the head with the idea in a deadpan sort of advertising way. Warhol used the screen printing medium to further accentuate the mass production/consumerism aspect. In fact, his studio was called "the factory," and he did little of the actual printing himself. Roy lichtenstein meticulously painted images to look like printed comic books (which were then in their heyday of printing).
