Endogamy is the practice of marrying those from the same religious, ethnic, or other social group. In most groups that have historically or currently practice endogamy, it has historically been enforced by social constraint or taboo.
Similar to cousin marriage, endogamy can change the number of possible maternal and paternal lineages in a population group. It can also change the expected number of autosomal DNA shared with cousins.
References & Suggested Reading: Wikipedia – Endogamy