This word is used to describe a class of people who fall somewhere between the lowest and highest classes. Bourgeoisie is often used insultingly.
In between the very poor and the super rich is the bourgeoisie. People have traditionally viewed the bourgeoisie as kind of crass and pretentious. The word yuppie could be a more contemporary synonym for bourgeoisie. In communist writing, the bourgeoisie are the capitalist class, as opposed to the workers (the proletariat). Most people in the bourgeoisie would probably deny that they are, just like many very rich people would rather say they are middle class.