No society gives absolute freedom to its members to select their partners. Endogamy and exogamy are the two main rules that condition marital choice.
It is a rule of marriage in which the life-partners are to be selected within the group. It is marriage within the group and the group may be caste, class, tribe, race, village, religious group etc. We have caste endogamy, class endogamy, sub caste endogamy, race endogamy, and tribal endogamy etc. In caste endogamy marriage has to take place within the caste. Brahmin has to marry a Brahmin. In sub caste endogamy, it is limited to the sub caste groups.
It is a rule of marriage in which an individual has to marry outside his own group. It prohibits marrying within the group. The so-called blood relatives shall neither have marital connections nor sexual contacts among themselves.