The F1 and F3 Green Cards are family based, first and third preference visas that provide lawful permanent residence to adult and married children of US citizens.
There is a limit to the number of visas available in these categories each year, so there is usually a waiting period before an immigrant visa number becomes available.
Immediate family members, such as spouses, parents and unmarried children under 21 years of age, of US citizens may be eligible for an IR Green Card which is unlimited and no waiting period applies.