The petite bourgeoisie are the social class that's made up of merchants and white collar workers. If your neighbor owns a small shop, she's a member of the petite bourgeoisie.
According to Karl Marx, the political theorist who popularized this French term in the nineteenth century, the petite bourgeoisie is made up of capitalists who operate on a modest scale. Inspired by the wealthier haute bourgeoisie, the petite bourgeoisie aspire to make more money and achieve a higher status. The term is a bit derogatory — in philosophy and literature they're portrayed as being spiritually empty and overly conventional. In French, the phrase means "little townsfolk."