Terrain roller coasters are roller coasters which, as their name suggests, take advantage of the (usually) natural undulations of the land upon which they are built. Such rides may often weave through forests, and some may even dive down cliffs. Because they tend to stay close to the ground, they require fewer supports and thus are usually cheaper than the same coaster on flat ground.
Famous examples:
Kennywood is well known for its hilly Pittsburgh location. Many of its coasters take significant advantage of the topography, for example, the Thunderbolt's last drop is its longest one.
Boulder Dash is an out and back coaster that traverses the side of a mountain
Holiday World is also known for its terrain roller coasters, most notably The Voyage.
The Beast at Kings Island has been the longest wooden roller coaster in the world since it opened in 1979. Only its two lift hills are visible from the ground or other attractions, the rest of its over 7000 feet of track is hidden below the treeline, deep in a forest.
Vortex at Canada's Wonderland is a terrain roller coaster, due to the influences of the ride track by Wonder Mountain and the river
