The middle class (or middle classes) comprises a social group once defined by exception as an intermediate social class between the nobility and working class folk. Since the working classes constituted the vast majority of the population, the middle classes actually lay near the top of the social pyramid. In Europe and the United States, industrialization eventually caused the middle class to swell at the expense of the lower, so that by the middle of the 20th century it constituted a majority. Now, the label is often swollen to cover the bulk of society and its norms.
As the swollen middle class lost its distinctive usefulness as a label, observers invented sub-labels: we often detect in contemporary societies at least an "upper middle class" and a "lower middle class".
Modern political economy considers a large middle class to be a beneficial, stabilizing influence on society, because it has neither the explosive revolutionary tendencies of the lower class, nor the stultifying greedy tendencies of the upper class.
For Marxist views on this class, compare bourgeoisie.