The proletariat is a class of people in Marxist theory. The proletariat is supposedly exploited by the bourgeoisie (capitalist class) and will therefore revolt in communist revolutions, implement the "dictatorship of the proletariat", and later a communist stateless utopia. See the Communism article.
The term derives from the Ancient Roman social class proletarii, which were Roman citizens owning little or no property. The derivation is arguably problematic, since the proletarii were not the lowest class in the Roman society, were not slaves, and had various rights and privileges that non-Roman citizens did not have.
In Marxist theory, there is also the "petite bourgeoisie", in the border zone between the proletariat and the "upper" bourgeoisie, but who supposedly often imitate the political views of the "upper" bourgeoisie.
Furthermore, Marx made a distinction between proletariat as salaried workers, which he saw as a "progressive" class, and the "Lumpenproletariat" ("rag-proletariat"), the poorest and outcasts of the society, such as beggars, tricksters, entertainers, buskers, criminals and prostitutes, which he considered a "retrograde" class.
Another problematic group to classify has been farmers, especially farmers owning land and not being paid wages, and who have often vehemently resisted Communist confiscation of their lands and produce, contributing to various mass starvations and mass killings of farmers by Communists.
In theoretical discussions, Communist parties have often struggled over the question of whether they should seek to organize and represent all the lower classes, or just the wage-earning proletariat.
In practice, however, Communist parties have adhered to principles such as "dictatorship of the proletariat" and "vanguard party" and have ruled dictatorially, ignoring the opinions of the proletariat, however defined.