From 2e's Complete book of Dwarves:
"Hill dwarves live in areas of rolling hills.
Their strongholds are primarily located underground, though they frequently have outposts on the surface. A typical hill dwarf stands 4 feet tall and weighs about 150 pounds. He is stocky and muscular. His skin is a deep tan or light brown in color and he has ruddy cheeks and bright eyes, His hair could be black, gray, or brown. He favors dark, somber, earth-toned clothes, and wears little jewelry.
Hill dwarves are the most common dwarves. They have adapted well to life above and below ground. They claim that they have always lived in the hills, but they may have migrated there either by traveling above ground, or via underground passages. If by surface travel, they are probably descended from mountain dwarves.
The alignment of the hill dwarves is usually lawful good, but there is no reason they cannot be of another alignment. So long as the
majority remain lawful good, strongholds of chaotic, neutral, or evil dwarves will not unbalance a campaign and will give it more flavor and variety."
That's literally all the info they're given as backstory. They're slightly smaller than mountain dwarves, but same exact stats so everything is cosmetic.
Ironically they're listed as the most "common dwarf" but everyone portrays mountain dwarves as the baseline.
I know later editions also tried to paint them as more "gregarious and welcoming" dwarves (comparatively) with 5e giving the subraces actual differences, but overall other than their aesthetic there's not much to differentiate the two.