The difference between "immediate" or "preference" relatives is that immediate relatives may obtain permanent residency at any time while preference relatives must wait until their "priority" date is available which varies depending on country of origin and other factors.
Immediate relatives are spouses of US citizens, children (under 21) of US citizens, and parents of US citizens.
Preference relatives are sons and daughters of US citizens, spouses and unmarried children of permanent residents, and brothers and sisters of US residents.
Immediate relatives are not allowed to bring other family members with them while preference relatives may bring along their immediate family.
