The Founding Families were the initial terraformers (PROSE: Legacy) from Earth that established colonies on other worlds. They generally held power over their colonies and served as the governing body.
In the Elysium system, the Founding Families maintained religious order and kept themselves secluded, which resulted in inbreeding to preserve their "purity." A civil war eventually broke out between rebels who wanted to resume contact with Earth and the Families that wanted to keep their power. (PROSE: Parasite)