Who are the “hired girls”? How are they different from other people in Black Hawk?

In Black Hawk, the "hired girls" are all daughters of immigrant farmers; they are country girls, as opposed to the town girls in Black Hawk.
These hired girls work to help their families pay off the debts accumulated from homesteading expenses. They work predominantly in the service industry. The text tells us that the Bohemian and Scandinavian girls cannot teach because of their poor skills in English. Instead, they work as housekeepers, waitresses, and store assistants.
The town girls believe that they are more "refined" simply because they do not work to earn an income. Generally, Black Hawk residents look askance at "hired girls," and these girls' moral values are often questioned. Hired girls are basically considered a "menace to the social order." They are spirited girls, and their beauty is more vivid than convention dictates. However, their industry is exemplary: these girls are directly responsible for the prosperity of their immigrant homesteading families in the story.
After their families are cleared from debt, hired girls marry farmers of like nationality. Then, they preside over prosperous farms, and their children are often better off than those of townsfolk in Black Hawk. In the story, Antonia, Lena, and Tiny are all "hired girls."