A culture is any group of people with shared behaviors. Different cultures have different traditions and customs, eat different types of food, speak separate languages or dialects or embrace different ideas of behavior. Most people participate in more than one culture at the same time. For instance, an Irish-American from South Boston may exhibit some cultural traits common to most Americans, some specific to Irish-Americans and some found only in South Boston. If this person married an Italian-American from a different neighborhood of Boston, he would probably notice some cultural differences with his partner. If he married a Midwesterner of German descent, the differences would probably be greater. If he married a Muslim of Iranian descent, the differences would be much greater. The term "multicultural marriage" usually refers to marriages in which the partners come from cultures with significant differences.

Some people use "multicultural" as a synonym for "interracial," but the two terms actually have different meanings. Culture describes behavior, while race describes biology. According to an article on the PBS website, science has found no evidence that different cultural behaviors are caused by biological differences between distinct groups of human beings. Two people from the same racial background can show more genetic diversity than two people from different racial backgrounds, so differences in race do not account for differences in culture. A marriage can be multicultural without being interracial or interracial without being multicultural.

When both partners share most of the same cultural assumptions, they are unlikely to find the minor differences a source of conflict. In the U.S., many people are married to people from slightly different cultural backgrounds without significant tension, because both partners share the experience of participating in a shared American culture even though they may have grown up going to different churches, eating different foods or celebrating holidays in different ways. When people from cultures with significant differences get married, each partner must grapple with an unfamiliar set of expectations and assumptions.

Different cultures value different things. The same behavior that one culture sees as frankness and honesty can be viewed by a person from another culture as rudeness and hostility. A person from a strongly family-oriented culture might find it only natural for his mother and father to help him make major life decisions, while a person from a more individualistic culture might see the same behavior as meddling. Some cultures value punctuality while others take an easygoing attitude about time and scheduling. One of the biggest challenges in a multicultural marriage is to avoid the tendency to see your own culture as being correct and other cultures as being wrong. Multicultural couples need an open, flexible attitude and strong communication skills to overcome these barriers.