U.S immigrant visas have different categories based on the intent of immigration. These categories are:

Family-Based Immigrant Visas to reunite with close family members;
Employment-Based Immigrant Visas to work in the U.S without an Employment Authorization Document (EAD);
Diversity Visas for those with low rates of immigration to the U.S;
Returning Resident Visas for those who have once held a U.S immigrant visa and want to return to the country again.
This article will go through the relevant information for Family Based Immigrant Visas.

What are Family Based Immigration Visas?
Family immigrant visas are for those who want to join their family members in the U.S. The U.S government though allows only close family members to unite. Close family members are considered spouses, children, parents and siblings. Other relatives such as grandparents or cousins do not qualify.