Popular American culture, like all popular culture, involves arts, trends and shared discourse that's available through common media and is accessible to a wide number of people, regardless of any privilege offered by social status, education or other specifics. Popular culture includes, but isn't limited to, movies and television programming, popular music and other entertainment, as well as references like cultural memes, jargon, trends and iconic symbolism.
Popular, or Pop, culture is usually understood as distinct from high culture or avant-garde culture. The latter two commonly imply a level of exposure or learning that grants only certain people access to them, something that's limited as opposed to popular. Popular culture, on the other hand, is that which is readily available and recognizable to the masses. Ashley Crossman at About.com defines it as that "accumulated store of cultural products such as music, art, literature, fashion, dance, film, television and radio that are consumed primarily by non-elite groups such as the working, lower and middle class."
Therefore American popular culture includes everything from reality shows to the latest pulp novel, songs atop the Billboard charts and the movies currently up for Oscar nods. Similarly, it can include slang phrases, celebrity knowledge and references to cult classic films. It can also be public caricatures of politicians or athletes, commercials and cola wars, hot-rods, haircuts, hand gestures and clothing styles. Popular culture is never static and depends greatly on the specific era under study. For example, 1950s popular culture would emphasize greasers, dances like the twist and B-movies about aliens. Alternatively, 1970s popular culture might center on hard rock and disco, the original Star Wars, leisure suits and bell-bottom pants.