The majority of immigrants to the United States have relatives already in the country who can petition to bring them here.  Only the following classes of relatives are eligible to immigrate:

There are long waiting lists in some of these categories, and the application process for each case is different depending on where the relative is living.  

Spouses, parents and unmarried children under 21 of U.S. citizens generally don't have a long wait for residence.

Adult and married children of citizens must wait several years; brothers and sisters of citizens must wait at least 10 years; spouses and children of permanent residents generally have to wait between 4 and 8 years.