What is meant in detail by "usul," "furu'," and "dhawu al-qurba"?

The term "ancestors" refers to those from whom a person is descended, such as the father, mother, grandfathers, and grandmothers, no matter how far back the lineage extends. They have specific rulings pertaining to them concerning maintenance and Zakat. As for "descendants," they are those who are descended from a person, such as sons and their sons, no matter how far down the lineage extends. "Relatives" (dhawu al-qurba) are one's kin from the father's or mother's side, such as paternal uncles and aunts, maternal uncles and aunts, brothers, sisters, and the like.