In sociological terms, a subculture is one that shares values, norms and behaviors within itself but does not share them with the entire population. However, it is very difficult to actually identify which groups really should be placed in this category.
In the United States, primary culture would just be called American culture. We generally all believe in things like free enterprise, democracy, and personal liberty. There are many groups that could be identified as subcultures within that culture.
For example, we have ethnic or racial subcultures. Mexican-Americans would be an example of this. We have religious subcultures. This would include groups like Mormons or evangelical Christians. We have subcultures that are based on occupation. These could include subcultures like those of members of the military or of the police. Finally, you can argue that we have subcultures based on shared interests like a hip-hop subculture or a Goth subculture.