Who are the proletariat? A lot of people assume that Marx is talking about all the poor, all the have-nots. But the proletariat is actually a more specific group: they're property-less, employed workers—property-less meaning that they have no possession of the means of production or capital, or spare money to invest. Marx isn't talking about houses or apartments or personal belongings here.
Picture a miserable factory worker out of a Dickens novel, and you've got yourself a proletarian. In the decade or so prior to the publication of the Manifesto, workers saw the promise of the Industrial Revolution—that people's needs would be met by all the new technology—remain unfulfilled. They were stuck working in horrible conditions in mines and factories. There were even food shortages, which led to riots. The proletarians were increasingly attracted to socialist and communist views, and they were a ready audience for Marx.
Marx and other communists wanted a revolution that would end exploitation and oppression forever—note that the preface quickly describes the proletariat as simply "the exploited and oppressed class" (Preface.6). But the problem was, previous changes—such as the bourgeoisie becoming more powerful than the aristocracy—had simply resulted in new types of exploitation and oppression. How would the revolution of the proletariat be any better?
Picture a kitchen table with a huge dog half-asleep under it. Sitting at the table are the aristocracy, the bourgeoisie, and others who are fighting to be in charge. The monarchs have their way of oppressing (feudalism), the businesspeople have their way (exploiting wage-labor), but the dog (the proletariat) is stuck eating the table scraps regardless of who's winning above.
Marx thought that because the dog doesn't have a seat at the table, it could learn that the table itself—the class division between haves and have-nots—was the problem. So even if the dog were to vote its way up to the table and secure its position there, the bigger problem would remain.
Here's big bad Marx in his own words:
All the preceding classes that got the upper hand, sought to fortify their already acquired status by subjecting society at large to their conditions of appropriation [ways people are ripped off or exploited]. The proletarians cannot become masters of the productive forces of society, except by abolishing their own previous mode of appropriation [i.e. except by ending wage-labor and the private ownership of the means of production]. They have nothing of their own to secure and to fortify, their mission is to destroy all previous securities for, and insurances of, individual property.
All previous historical movements were movements of minorities, or in the interests of minorities. The proletarian movement is the self-conscious, independent movement of the immense majority, in the interests of the immense majority. The proletariat, the lowest stratum of our present society, cannot stir, cannot raise itself up, without the whole superincumbent strata of official society being sprung into the air. (Section2.48-49).
Workers of the World, Unite!
So, imagine that the dog rises up and knocks over the table and those sitting in the chairs, ending class struggle forever. Although he didn't make this idea explicit in the Manifesto, Marx argued in The German Ideology and The Poverty of Philosophy that the revolutionary struggle itself—the dog struggling to rise up and knock over the table—would teach the proletariat how to prioritize the common good and make the post-capitalist, communist world a good one. The already-unionizing workers would fight side by side and learn to value sharing and each other.
However, some people misread the Manifesto and see Marx as an advocate for all the poor, all the have-nots. But Marx's quick one-paragraph (Section1.46) dismissal of the lumpenproletariat makes it clear that by proletariat he specifically means employed workers.
After all, elsewhere in the Manifesto, he defines the proletariat as "a class of labourers, who live only so long as they find work, and who find work only so long as their labour increases capital. These labourers, who must sell themselves piecemeal, are a commodity, like every other article of commerce, and are consequently exposed to all the vicissitudes of competition, to all the fluctuations of the market" (Section1.30).
So, the proletarians are the ones who are getting the short end of the stick in the marketplace. Labor that's not commodified (bought and sold)—say, hand-making an earthenware bowl and gifting it to your friend, or giving birth to a child—isn't proletarian labor by the terms of the Manifesto. See our discussion of the lumpenproletariat or the "Steaminess Factor" section for more thoughts on this.
Marx seems to think the proletariat is the ideal group to overthrow the bourgeoisie because it has no means of production or capital to safeguard. He also seems to take for granted that the proletarians would make good revolutionaries because a) they are massive in number ("Working men of all countries, unite!"), and c) they undergo shared suffering either in the workplace or during the revolutionary struggle.
Did Marx romanticize the proletariat? What do you think, comrade?
The Proletariat's Timeline