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The proletariat (often shortened, "prole" in slang) is a term used by communists for a segment of society. It describes the class of workers in urban areas who work in industry or manufacturing. These urban workers are different from the peasants, who are workers who do farming jobs in rural areas.
The proletariat is the laboring bulk of society that does most of the useful, necessary work, and pays most of the taxes. It is also perceived by "capitalists" (another communist term) as being necessary for those reasons: labor and taxes, plus being available to exploit to produce profits.