We define a culture as a set of characteristics that sets one group of people apart from another. For example, we sometimes attribute differences in the way the English act as opposed to the French as the result of "cultural differences." In doing so, we take a vast number of differences and agglomerate them under one umbrella.
Some characteristics are transcultural, though. For example, we more or less expect that all children, everywhere, will grow up to love their parents. The way they express that love may depend a lot on the culture in which they are raised, but the loving feelings are culturally agnostic.
Another way to think about this is that a culture defines how a set of abstract principles is translated into day-to-day behavior. That is, we all have a set of nearly instinctive "default behaviors," programmed into us from infancy, which represent accepted norms and modes within our local environment. Of course, we can consciously choose to behave outside these norms, which we may well do in unusual situations. For example, if we live in a culture that believes violence is bad, then when disagreements arise, our default behavior, according to that cultural norm, is to use our words and not our fists--to resolve conflict verbally. However, if it becomes clear that someone who is threatening another person's life will not "listen to reason," then the culture admits violence as a "last resort" for the potential victim--or a law enforcement officer. But this is an exceptional case. In general, there are sanctions, both formal and informal, for violating cultural norms when exceptional circumstances do not apply.