Because man is a social creature (he seeks to establish relationships to others as ends or means), he affects and is affected by many who he comes in contact with. His way of thinking, acting, behaving are shaped by the responses or lack thereof of those around him. In addition, man seeks to know truth and he recognizes that others seek to know truth. Because opinions of truth differ and ability to see ‘truth’ differ, man will often seek to convince others of the truth of his perceptions. Insofar as he convinces them, his perception of truth, which they now adopt, may influence their actions, thoughts, behaviors, etc.
The product of this dynamic is a network of people connected by interpersonal relationships often based geographically but also by race, sex, social class, religion, etc., such that people think, judge, and act similarly in what has come to be known as “culture”. Culture is “the (common) way of life” of people of a specific (most generally incorporating a geographic) characteristic. Culture includes similarities both in what is thought and what is not thought about; what is done as a matter of course and what would never be done, as a matter of course. It ranges from (a common practice of) commission to (a common practice of) omission.
A related concept is “custom”, which involves a regular practice accepted by society. The difference between culture and custom is that “culture” incorporates a much more interconnected, expansive, and all encompassing “way of life” that may unknowingly pervade the most fundamental ways of thinking. “Custom” on the other hand is generally a practice that, conceivable, is independent enough to adopted by another group without a submission to the whole culture of the first (not denying the fact that some of the cultural things may come along with it).