This category is limited by quotas. There are different waiting periods for natives of different countries. For information on your waiting period, please visit http://travel.state.gov/visa/frvi/bulletin/bulletin_1360.html. There is a longer waiting period as the preference descends in scale. In other words, the second preference has a longer quota than the first preference and so on.

• Family first preference: Unmarried persons, any age, with at least one U.S. citizen 
  parent.
• Family second preference: This category is further split into two divisions:
         • 2A: Spouses and children under the age of 21, of green card holders;
         • 2B: Unmarried sons and daughters of green card holders who are at least 21
                 years old.
• Family third preference: Married people of any age, related to at least one U.S. citizen
  parent.
• Family fourth preference: Sisters and brothers of U.S. citizens, where the U.S. citizen 
  is at least 21 years old.