Roman citizens were divided into two classes, Plebeians and Patricians.
The plebeians were the lower class. They included everyone who was not a patrician. They were sometimes just called plebs.
The patricians were the upper class. They were the wealthy land owners.
All free adult males were citizens, no matter what their class. In both classes, the oldest male was the paterfamilias or head of the family. Old age was honored, and women had no rights. If you could afford them, both classes owned slaves. The houses of both classes were designed the same way unless you were too poor to own your own home. Everyone worshipped the same gods, and observed the same festivals and holy days. Everyone spoke Latin and everyone went to the baths and enjoyed the forum.
Plebeians and Patricians rarely mixed socially. For a while, it was illegal for a pleb and patrician to marry. Under the Republic, that law was finally changed. Still, marriages between the classes was rare.