From WoWWiki

Hill dwarves is a term used for clans of dwarves that live primarily in the hills. They construct their buildings above ground, with roofs opened to the sky (rarely digging very deep into the hills). The primary clan of hill dwarves is the Wildhammer Clan.^{LoC 19}

However, not all hill dwarves are affiliated with the Wildhammer dwarves or their lesser clans. Some hill dwarves live in Khaz Modan and share an old emnities with their wild dwarven cousins. They seem to be connected to the mountain dwarves of Khaz Modan. When the Horde attacked Khaz Modan, most of the dwarves escaped (including some of the hill dwarves), however most of the hill dwarves escape routes were cut off and they were stuck behind enemy lines. They were forced to go underground more so than they previously had done before. Many had died at the time, and more died while the Horde held the land. Only four clans of hill dwarves were left in Khaz Modan. One clan (Rom's clan) was left with fourty-seven members where once numbered in the hundreds. Two of the clans were larger than the first, and the fourth was either the same size or smaller than the first. Between the four clans, the number of individuals totalled three-hundred and a little over, which was only a small fraction of the number they once were in the land.^{DotD 259} A clan of hill dwarves helped take back an area near Grim Batol during the Second War.