"The proletariat is that class in society which lives entirely from the sale of its labor and does not draw profit from any kind of capital; whose weal and woe whose life and death whose sole existence depends on the demand for labor -- hence on the changing state of business on the vagaries of unbridled competition. The proletariat or the class of proletarians is in a word the working class of the 19th century." [Quote p.81]
According to Engels the working classes have according to the different stages of development of society lived in different circumstances and had different relations to the owning and ruling classes."In antiquity the workers were the slaves of the owners ... In the Middle Ages they were the serfs of the land-owning nobility ... there were also journeymen in the cities who worked in the service of petty bourgeois masters." [Quote pp. 81-97]
Later Engels delineates the differences between slaves serfs and proletarians -"The slave is sold once and for all; the proletarian must sell himself daily and hourly ... The serf possesses and uses an instrument of production a piece of land in exchange for which he gives up a part of his product or part of the services of his labor ... The serf has an assured existence the proletarian has not. The serf is outside competition the proletarian is in it. [Quote pp. 81-97]