First of all, “immediate relatives” of a USC, including parents, spouses, widows, and children of a USC (children who are unmarried and under 21 years of age, and, in the case of a parent of a USC, the petitioning son or daughter being at least 21 years of age) can immigrate to the United States without being subject to any numerical restrictions. They can apply for the permanent resident status without any waiting time. The rest of the Beneficiaries are divided into several groups called Preferences. Each Preference is given a numerical quota per year to limit the number of immigrants admitted into the United States.
The four Preferences are as follows:
1st Preference: applies to unmarried sons and daughters of U.S. citizens
2nd Preference: applies to spouses and unmarried sons and daughters of lawful permanent residents
3rd Preference: applies to married sons and daughters of U.S. citizens
4th Preference: siblings of U.S. citizens