http://travel.state.gov/visa/immigrants/types/types_1306.html
Overview - Family-based Immigrant Visas
Two groups of family based immigrant visa categories, including immediate relatives and family preference categories, are provided under the provisions of United States immigration law, specifically the Immigration and Nationality Act (INA).
Immediate Relative Immigrant Visas (Unlimited): These visa types are based on a close family relationship with a United States (U.S.) citizen described as an Immediate Relative (IR). The number of immigrants in these categories is not limited each fiscal year. Immediate relative visa types include:
IR-1: Spouse of a U.S. Citizen - Learn More
IR-2: Unmarried Child Under 21 Years of Age of a U.S. Citizen
IR-3: Orphan adopted abroad by a U.S. Citizen - Learn More
IR-4: Orphan to be adopted in the U.S. by a U.S. citizen - Learn More
IR-5: Parent of a U.S. Citizen who is at least 21 years old
Family Preference Immigrant Visas (Limited): These visa types are for specific, more distant, family relationships with a U.S. citizen and some specified relationships with a Lawful Permanent Resident (LPR). There are fiscal year numerical limitations on family preference immigrants, shown at the end of each category. The family preference categories are:
Family First Preference (F1): Unmarried sons and daughters of U.S. citizens, and their minor children, if any.
Family Second Preference (F2): Spouses, minor children, and unmarried sons and daughters (age 21 and over) of LPRs.
Family Third Preference (F3): Married sons and daughters of U.S. citizens, and their spouses and minor children.
Family Fourth Preference (F4): Brothers and sisters of U.S. citizens, and their spouses and minor children, provided the U.S. citizens are at least 21 years of age.
Note: Grandparents, aunts, uncles, in-laws, and cousins cannot sponsor a relative for immigration.