A segmentary lineage society is characterized by the organization of the society into segments; what is often referred to as a tribal society.
A simple, non-anthropologist's explanation is that a close family is the smallest and closest segment, and will generally stand with each other. That family is also a part of a larger segment of more distant cousins and their families, who will stand with each other when attacked by outsiders. They are then part of larger segments with the same characteristics. Basically, if there is a conflict between brothers, it will be settled among all the brothers, and cousins will not take sides. If the conflict is between cousins, then brothers on one side will align against brothers on the other side. However, if the conflict is between a member of a tribe and a non-member, then the entire tribe including distant cousins could mobilize against the outsider and his or her allies. This tiered mobilization is traditionally expressed e.g. in the Bedouin saying: "Me and my brothers against my cousins, me and my cousins against the world."
The ancient Hebrew nation (the Israelites) is one of the better-known examples, with biblical tradition denoting 12 tribes originating from one common ancestor (Jacob). The largest segmentary lineage society today is believed to be the Pashtun people of Afghanistan and Pakistan, with some 50 million members organized into a vast tribal structure. Arab tribal officials say that Bani Tamim, an Adnanite Arab tribe, has more than 42 million members, who all trace their lineage back to one man called Tamim.