Literary genre, c. 1910-1950
Plucky girls were generally eight to twelve: that is, old enough to know a few things, but without the turmoil of puberty. They were always New Kids in Town, having been orphaned, ill, or otherwise displaced (if the story is set in San Francisco, she's from Vermont, on a farm, if the story's set on the Prairie, she's a city slicker from New York, etc.) Very rarely, they might be there with her birth family (Caddie Woodlawn) but more often, she's trying to adjust to living with strange-to-them relatives or guardians as well as changed circumstances. Often described as "tomboyish", they have random adventures, befriend people different from them in race, class, sex and age, get into real trouble, and end up doing something heroic that proves their assimilation by the end of the book. Oh, yes, they also learn to wear pink dresses and make friends with the guy who's going to be their first love in a few years.
Sound Mary-Sueish? Well, kind of, unless you count the fact they screw up (and how!) at least once in the book.