The proletariat (from Latin proles, offspring) is a term used to identify a lower social class; a member of such a class is called a proletarian. Originally it was identified as those people who have no other wealth than their sons; the term was initially used in a derogatory sense, until Karl Marx used it as a positive term to identify the what he termed the working class.
In Marxist theory, the proletariat is that class of society which does not have ownership of the means of production. Therefore, the only source of income for proletarians is wage labor.
Marxism sees the proletariat and Bourgeoisie (owner class) as inherently hostile, since (for example) factory workers automatically wish wages to be as high as possible, while owners wish for wages (costs) to be as low as possible.
See also: Wage slaveryThe Proletariat in Marxist theory