I don't really know how the social classes work in the rest of the world, but in the United States we suffer under the curious delusion that there is a "middle class" of happy folks who are living the American dream. I'd like to dispute that.
The idea of a middle class is a corruption of the idea of class itself. Now that we've done away with silliness like divinely-sanctioned royalty, we're left with only two essential classes in capitalist society: capitalists and workers. Good ol' Marx's bourgeoisie and proletariat. These classes are formally based on the members' relationship to the means of production, but the easiest way to think about it is to ask, "Where does their money come from?"
Capitalists make their money and living off of others. They hire people. They own businesses and equipment. They rent out land and loan out money. They could, if they chose, not lift so much as a finger in actual work themselves and they would continue to be able to afford food, shelter, clothing, medical care, and all the necessities of life. They don't have to work.
Workers, on the other hand, make their money by seeking employment from capitalists. They sell their labor. It may be industrial labor, it may be mental labor, but it is labor all the same. Unlike the capitalists, if the workers chose not to lift so much as a finger in actual work, they would starve to death. They have to work.
You'll immediately note that these categories are not cleanly cut and easy to see. There are small business owners who are capitalists but barely scrape by, and there are extremely well-paid engineers and the like who, nonetheless, couldn't quit their jobs and make a living off investments. When it comes to class, money is not the issue. That money is unfairly distributed is true, but it is only a symptom of the problem of class-based society.
Ultimately, the class distinction is nothing more than a reflection of our respect for authority. Rather than a just system of possession, we have a society based on private ownership of everything. Capitalists own businesses, and profit from businesses, but they do not make the profit themselves. If the capitalist disappeared, the business could continue and the workers could keep the profit they earned. If the workers disappeared, the capitalist would be out of luck, as no profit would be made. What keeps the workers from taking the profit they earn is nothing more complex than words (a claim of ownership) backed up by force (police).
So what about the middle class? Well, clearly most middle class people are still workers. The idea of the middle class has one purpose: to make middle class folks think they're superior to the working class people who might be poor enough to get upset and demand changes. Middle class people are supposed to be content, and not want to rock the boat. The invention of the middle class is a means of dividing the workers, by allowing one segment of the group to have a few more crumbs than the rest, driving a wedge of jealousy on one hand and apathy on the other between people that should be united against their common foe. The invention of the middle class is designed to convince its members that capitalism is OK, simply because it is apparently OK for them at the time.
The artificial line between the working class and middle class, often trotted out during election season, is one that should be forgotten. It is not the fault of the middle class that the capitalists chose their positions as those that would earn more, and it is not the fault of the working class that the capitalists didn't choose theirs. All people who must sell themselves for the benefit of another, for fear of death, ought to be united against those who would force that choice upon them.