There are two types of family based immigration petitions. The first category is “immediate relative” and the second category is “preference relative” petitions.
The immediate relative petition category covers circumstances where the petitions is a US citizen over the age of 21, who is petitioning for a spouse, parent, or child who is unmarried and under age 21.
Other immigrants will be included in the preference categories and include the following relatives. (1) Unmarried, adult (over 21 years old) sons and daughters of U.S. citizens; (2) Spouses of lawful permanent residents; (3) unmarried children of lawful permanent residents; (4) unmarried adult (over 21 years old) sons and daughters of permanent residents; (5) married sons and daughters (any age) of U.S. citizens; (6) brothers and sisters of adult (over 21 years old) U.S. citizens.
The main difference is that immediate relatives will not be subjected to a waiting period for a visa, and can, in certain circumstances, immediately adjust status to lawful permanent residence if the immigrant is already in the United States. Preference relatives will be subjected to a waiting period for a visa, and will have to wait to apply for adjustment to lawful permanent residence.