The true origin of the distinction between plebeians and patricians is unknown; there is little evidence for any sort of a racial basis, nor many signs of a distinction during the time of the kings. In any case, around time of the foundation of the Roman Republic, the plebeians were excluded from religious colleges and magistracies, and the law of the Twelve Tables disallowed intermarriage. At the same time, plebeians were enrolled in the gentes and tribes, served in the army, and could become military tribunes.
Even so, the "Conflict of the Orders" over the political status of the plebeians went on for the first two centuries of the republic, ending with the formal equality of plebeians and patricians in 287 BC. The plebeians achieved this by developing their own organizations (the concilia plebis), leaders (the tribunes and plebeian aediles), and as the ultimate weapon used the secessio, by which the plebeians would literally leave Rome, effectively boycotting the city. This is recorded to have happened five times, although only the last (in 287) is believed to be accurately documented.
Later on "plebeian" came to mean the poorer members of society in general. During the Empire it was often used of anyone not in the senatorial or equestrian orders.