Social class in the United States is a tricky thing to discuss, in large part because most Americans are in denial about its existence. But even though it's something nobody talks about, it exists and affects almost every aspect of people's lives, from the schools they attend to the clothes they wear.
Class is mostly about money, but not about the amount which you personally have. Rather, it's about the wealth of your family and how you were brought up. While people can change their class, for the most part this is only possible during one's childhood and adolescence. By early adulthood, one's class is set, and is almost impossible to change. This is largely why fancy/elite schools exist: they train children in how to adopt the customs and class markers of the upper middle or upper class. (This is also why people from higher classes care so much about their children's education; if they are sent to a low-class school, the children's class status might slip.)
The near impossibility of changing one's class after age 20 or so can be seen in the existence of the "nouveau riche", who are people with enough wealth to belong to a high class, but who lack the culture and class markers necessary for membership. The opposite, of the "decayed gentry", or people who are poor but in a higher class than their incomes would suggest, demonstrates how permanent class is, even late into adulthood. (Since most of these people are intellectuals, it also shows how important education is to class.)
There are four main classes in the United States:
--Upper Class: those who don't need to work; their income comes from investments, property, mineral rights, etc. There aren't a lot of these people in the US, and most of them live in only a few places (certain areas of the East Coast, San Francisco, a few other large cities). Most of them are WASPs, which in addition to connoting a certain race, ethnic background and religion describes a particular culture. Important elements include the outdoors (WASPs tend to love expensive, non-team sports like golf, tennis, sailing and skiing) and understatement (no obvious brand labels, muted colors, carefully distressed furniture/clothes). The upper class tends to be taller and thinner than average (in general, Americans' weight is highly correlated with their class: the higher the class, the slimmer), and also relatively stupid (the brightest Americans are usually middle class). Despite their relative lack of brains, this class has a great deal of power (owning most companies, for example), but they don't usually do the hands-on work (George W. Bush is a good example of this in action), which is reserved for the middle class.
--Middle Class: those who work at skilled jobs. Unlike the upper class, they must work to support themselves; however, those at the top of this category (the Upper Middle Class) can make very large salaries indeed: most CEOs, doctors, lawyers, and investment bankers are here. Because the middle class doesn't tend to have a huge amount of inherited wealth, education becomes hugely important in maintaining one's middle class status. This class dominates American society culturally (and demographically), with the upper middle class in particular in charge of day-to-day operations in almost every area.
---Upper middle class: these are highly skilled professionals, often with tertiary degrees. Most of them live in urban centers. To some extent, their tastes and habits mimic the upper class's (in part because they may attend the same schools).
---Middle class: skilled professionals, who usually have college degrees. Their jobs don't offer the same level of autonomy and responsibility as the upper middle class (nurse vs. doctor, elementary school teacher vs. college professor) but are usually fairly well-paying. They like TV, American food and branded products better than the upper middle class. They are also more religious.
---Lower middle class: skilled workers, who may or may not have higher education (store supervisors, skilled tradesmen). This is the class hardest hit by the growth of the new American economy, as well-paying union jobs disappear. If they are lucky, they can get more education and move upward; otherwise, they end up in the working class.
--Working class: those who work at unskilled jobs, not requiring much education. Examples would be waitress, store clerk, or factory worker. Their jobs don't pay that well, usually have unpleasant working conditions (like numerous punitive rules, rigid working hours or no benefits), and may be unsteady/unreliable in working hours. They tend to be less puritanical, and less uptight, than the middle class. They also tend to value education less.
--Non-working class: those who do not have regular jobs (many of them do work, but off the books, at illegal activities, or only seasonally). Being on welfare is a common source of income. This group has a disproportionate number of social problems, including a large number of its members in jail (partly because they commit more crimes, partly because the other classes are more savvy at not getting caught).
The best description I have read of the different social classes' tastes, pursuits, and values is in Paul Fussell's Class in America. It's a little old (from the nineties I think?) but still mostly relevant.