Yeah, Dollar Tree is, well, a junk store. Everything is one price (one dollar, which means that inflation means the merchandise gets lower in quality every year). It is just junk. Leftovers, arts and crafts crap, coloring books, whatever.
Family Dollar and Dollar General are poor people's stores. Selling basic items like staple foods, housewares, cleaning supplies, and basic clothes for low prices. In the country, this can help people make ends meet, and they really have an impact on poor neighborhoods where many people have limited transportation. In the city I work in I have seen these stores make a difference in poor neighborhoods.