The 10 Great Families are the direct descendants of the 10 Great Warriors who first came into the Tower. They are part of the Ruling Class in the Tower, and their members are often extremely powerful, blessed with the power of their forefathers, meaning that many of them are Rankers. Just as Zahards are considered Royals, 10 Great Family members are considered Nobles.
Another interesting point about these 10 Great Families is that a few selected daughters of the family have the chance to become Zahard's Princesses, and gain an enormous authority and power in the Tower. On this site the 10 great families will be non-canon names unless some of the canon ones are requested.