A plebeian, or pleb, was a average Roman citizen, usually thought of as lower class. Though a citizen, a plebeian did not enjoy the status and rank of the higher-class Romans, namely the Patricians.
Early on in the Roman Republic, the plebs were not allowed to participate in religious ceremony, politics, and could not marry Patricians. In 287 B.C., when the plebeians and Patricians came to an accord of equality and harmony, the plebs were granted sort sort of level citizenship, and could enlist in the military, participate in the state religions, and intermarry.
Eventually, plebs could participate in politics, even become Senators. By the late Republic, plebs had become among some of the wealthiest Romans, and the lines between the once lower class plebs and the elite patricians blurred.