Family Based Immigration
There are two types of family-based immigrant visa categories - Immediate Relatives and Preference Categories.
The first group is the immediate relatives category, which includes, spouses or US citizens, parents of US citizens, if the US citizens are over 21, and children of US citizens, who are under 21 and unmarried. This category has no annual numerical limitation.
The second group is the Family Preference Immigrant Visas. This visa is subject to numerical limitations and are available to unmarried and married sons and daughters of US citizens, spouses and children of Permanent Residents, unmarried sons and daughters of Permanent Residents and siblings of US Citizens, if the US Citizens are at least 21 years old.