You get $6 an hour folding T-shirts at Abercrombie. You complain about the side effects of breathing in that much perfume and demand a raise. Boss says no. You go on strike. Boss fires you. CLASS STRUGGLE. For Marx, the conflict of interest between employers and employees is what history is all about.
All the stuff that you can turn a profit from. In the olden days, this meant factories, railroads, and Scrooge McDuck-style piles of gold. Nowadays, we're talking things like stocks and software patents. At the most basic level, when you use money to get more money—that's capital.
Whatever seems the most natural, sensible, and good way of behaving—that's your ideology right there. Recycling? Ideology. Charity? Ideology. That slogan on your T-shirt? Ideology. That ironic slogan on your t-shirt that's actually quite funny when you think about it? Ideology. Chances are you didn't make that ideology up yourself—you got it from somewhere else… probably from someone or some group of people who would like to tell you what to do.
Intellectual or artistic products that mean something. Most Marxists will tell you that culture is a cover for ideology—that is, it's a way for people to feed you their political message without you necessarily knowing they're doing it. Music, literature, art, film—Marxists think that they usually say more than we realize. Even mindless entertainment is hiding some sneaky ideas up its sleeve. That's right: even Miley Cyrus—or at least her record company—is trying to control your mind.
This is a dance craze Marx invented when he was best man at Engels's wedding. Okay, it is not. It's actually a way of thinking by means of unifying opposites. As a theory of history, dialectical materialism tells a story of the world as the continual resolution of contradictions. The conflict between employer and employee? That's a conflict that gets resolved in lots of different ways, all the time, and Marxists think that's what makes history happen.
None of this airy-fairy art for art's sake stuff for Marxists; they think art should change the world. Commitment to the Marxist cause means that Marxist artists should try to spread their message via art. Give that poem a message, folks! Give that song some relevance! Take a stand with that amusing This American Life story about a policeman who chases a squirrel with a flashlight and sets fire to a sofa. Act One: Class Struggle.
Yay, workers! Downtrodden but down to earth, overworked but underpaid, worldly-wise but long-suffering—these boys and girls are the salt of the earth and will one day rule the world. But let's get a little specific: for Marx, proletarians aren't just any old workers; they're specifically urban laborers, and a lot of what the dude had to say about class struggle comes from the struggle he witnessed between these urban laborers and the bigwigs who exploited them. Which brings us to…
Boo, employers! Hiss! Exploiters! Bloodsuckers! These meanies own the means of production and their patron saint is The Simpsons' Mister Burns. More specifically, though, the bourgeoisie is basically the middle class, or the upper middle class: people who own stores and factories and, as Marxists see it, make lots of money by exploiting laborers. (These days in the West, most people are technically bourgeois: not too rich, but not poor, and living a comfortable lifestyle made possible by the cheap labor supplied by poorer countries.)
This used to just mean manpower, but these days, for example in corporations that depend on human resources to do their thing, it can also mean brainpower. Basically, if you want to use your capital to get rich and buy yourself a monocle and top hat, then you need workers, and specifically the products of their labor, to put it that capital to use. People are sort of like investments: you pay them a little money to make something that you then sell for way more money, and voilà, you're rich!
First there were hunter-gatherers, then there were farmers, next there were factory workers, now there are Apple Geniuses. That's progress, folks: the Iron Laws of History. According to Marxists, one day all this progress will get us to Socialism, which is what happens when the workers say they've had enough with this whole exploitation thing. Finally, when things are perfect, we'll have Communism, when everyone shares the means of production and no one has more–or less–than anyone else.