Rabbinic law as derived from the Talmud is binding upon all Jews. In addition, there have always been local customs and prohibitions, which are binding only where practiced. Indeed, the Talmud gives detailed rules for people who visit or move to a locale where the custom differs from their own. Custom can also determine halachic practice in cases of disagreement among rabbinic authorities.
Some customs were eventually adopted universally or almost universally (e.g. monogamy). Others are observed by some major segments of Jewry but not by others (e.g., not eating rice on Passover), while still others are practiced by specific subgroups (e.g., Lubavitch), locations (e.g., minhag Yerushalayim), or even families.
Jews whose ancestors originated in most areas of Europe (regardless of where they live now) tend to follow Ashkenazic customs, while those originating from the Mediterranean countries and the Middle East generally follow Sephardic customs. (“Ashkenaz” was the medieval Jewish name for a part of Germany; “Sephard” for a part of Spain and Portugal). A rabbi or yoetzet halacha should always be made aware of which customs are followed by the person asking a question.
