The most important difference between primary and secondary groups has to do with:

The most important difference between primary and secondary groups is that, primary groups are being referred to a family in which every person has while the secondary group is being referred to the society or the sociology class in which every person partakes to. This group has to do with the relationships that exist amongst them.

The important difference of primary and secondary group has to do with the kings of relationships that exist within them. Your family is the primarily group and the secondary group would be a sociology class. The group has to do with the relationship that exist within them.