An American citizen may petition for an immigrant visa for his or her spouse, parents, children, and siblings. A permanent resident alien may petition for immigrant visas for his or her spouse and unmarried children.
The spouse, parents, and unmarried children under age 21 of American citizens are not subject to annual numerical limitations. This means that such persons need not wait in a queue before their immigrant visa is processed. Married children of a U.S. citizen, unmarried children over the age 21 years of a U.S. citizen, siblings of U.S. citizens, and the spouse and children of permanent resident aliens are all subject to numerical limitations. There are only a limited number of such visas issued each year, and a surplus of applicants. The result is a waiting list. In some categories, the wait is many years. These visas are processed in the order that the I-130 petitions are filed. The filing date is called that applicant’s “priority date.” Once the priority date of the approved I-130 petitions becomes eligible for processing–or “current”–the applicant will be sent the detailed list of supporting documents needed to apply for the immigrant visa.
Resources on the U.S. Citizenship and Immigration Services Website