The highest social class of Ancient Rome
The patricians were the wealthest of the ancient romans. Partricians were able to trace their ancestry back to members of the original senate established by Romulus at the founding of Rome. Originally only patricians were able to hold political office but this did change over time.
The wealthiest of the patricians, worth over 1,000,000 sestertii, made up the senatorial class. Proof was required by the census of the wealth before someone of this class could take their seat in the senate. Generally members of this class did not engage in commercial activities. The senatorial class's wealth was often in the form of large farms outside of Rome.
Below the senatorial class was the equestrians which were worth 400,000 sestertii or more. This class of ancient romans were the merchents and business men of the era.
Still roman citizens but not of the highest social class
The plebs were the bulk of roman citizens, they just didn't have the approriate ancestry. They could, and often did, become wealthy. Initialy they could not hold any political offices, but after several plebeian secessions they began to get concessions from the patrician class, like the Tribune of the Plebs.
In the middle of the Roman Republic if a pleb was able to achieve enough wealth and political status, particularly election to the office of consul, they could move up to being a noble, called a 'new man'. Late in the Roman Republic there was very little actual difference in the social classes of patrician and plebs.
Freedmen and slaves
Not even a Roman citizen
Ancient Roman Slaves were the property of their masters. The slave owner could treat and day with their slaves anything they wanted, included killing them, which was not a crime. Ancient Roman society did considered it to be in poor form to be excessively cruel to ones slaves. Many slaves would have had a hard time of it working as field hands on the large senatorial farms or in mines working endless hours.
Many ancient roman slaves were highly trained and skills members of a household, often being considered an extended part of the family. These slaves were often close personal servants to their masters and were given some additional freedom not afforded the typical laborer. These slaves, if thrifty enough, could save up the coins they collected and potentially purchase their own freedom. A more likely way for a slave to achieve their freedom was through manumission, where the slave owner would free them for some reason. Some masters would free their slaves upon their death.
Freedmen were people that had previously been a slave but were now free. Where the child of a slave was a slave, the child of a freedman was a full roman citizen. It was difficult, but not impossible, for the decedent of a freedmen, or former slave, to climb up the social classes of Ancient Rome. Their best chance of social advancement would have been through service in the Roman military where they could potential earn a farm in conquered territory.
Social Class of Ancient Rome didn't mean everything, just almost everything
The structure of the social classes of ancient Rome did fluctuate and change over time. The descriptions of the social classes in this article most closely depict the classes of the mid to late republic period, 200 BCE to 50 BCE.