The Hinterlands are located just north of the Arathi Highlands. It's a large, mountainous, wooded zone, with a beach on the eastern edge. Alliance players enter from the western border, and immediately find themselves at the imposing home of the Wildhammer Dwarves, Aerie Peak, which is built into the side of a mountain.
Horde players enter from the south, and their base of operations here is at Raventusk Village, the home of some native trolls on the southern part of the beach.
The main antagonists in the Hinterlands are a separate group of trolls who are encroaching upon the two factions' hubs. They are coming from their massive ruined city of Jintha'Alor...
and have set up small camps throughout the zone, and the large Altar of Zul in the center of the southern portion of the map for all kinds of weird voodoo rituals.
There is also a small elvish presence here, the Quel'Danil Lodge is located in the center of the zone.
Finally, on the Northern border is another large clearing with a portal to the Emerald Dream, similar to Duskwood's, which also sometimes has a large dragon lurking.