The proletariat (from Latin proles, offspring) is a term used to identify a lower social class; a member of such a class is proletarian. Originally it was identified as those people who had no wealth other than their sons; the term was initially used in a derogatory sense ("those people only good for breeding"), until Karl Marx used it as a sociological term to refer to the working class.
Marxism defines the bourgeoisie as the social class which obtains income from ownership or trade in capital (economics) assets, or from commercial activities such as the buying and selling of commodities, wares and services. In medieval times, the bourgeois was typically a self-employed proprietor, small employer, entrepreneur, banker or merchant. In industrial capitalism, on the other hand, the bourgeoisie becomes the ruling class - which means it also owns the bulk of the means of production (land, factories, offices, capital, resources). This enables it to employ and exploit the work of a large mass of wage workers (the working class), who have no other means of livelihood than to sell their labour to property owners.