From the earliest beginnings of Rome the basic unit of Roman society was with family. Within the family units the father was to all intents and purposes "owner" of his wife, children, slaves to the same extent that he owned a sword and other material possessions. The head of the family could do whatever he wished to the family. Including selling them off as slaves or putting them to death.
Plebians
The plebs were the vast majority of society and they constituted a social class made of common folk. Generally speaking they were decedents of people who had come to Rome after the Patricians family had founded it. Possibly they were descended from freed-men or simple immigrants.
Slaves
Everyone knows what a slave is, and in early days Rome these people were largely confined to working in his house. The lord of the house had the same rights as he did over his wife and children. He could make slaves into free-men or can sell one of his family members into slavery.
Liberti
"Libertus" was the social class of a free slave. Though at first sight this might give impressions of someone doomed to a lowly existence there is plenty of evidence of liberti with great wealth.