There are seven terrain types in Evernight:

Notice the different icons for Dunes, Swamps, Dead Forests, and Mountains. Water (aka, Frozen Sea) is indicated by being filled with a blue color.

The Black Between is filled with a solid black. Mountains, in addition to having an icon, are always filled in with a dark brown color. Plains regions have no icon or fill color.

Terrain type and it's distinction versus the other terrains:

Plains

Produces The Most Treasure

Swamps

Can Not Build Structures

Dead Forests

Enemy Lessers and Woodens Hidden

Dunes

Each lesser has a 10% chance of dying each turn spent in dunes

FrozenSea

Passable Only By Iron Hulks, Black & Amber Dragons

Mountains

Passable Only By Iron Hulks, Black & Amber Dragons

Black Between

No forces or structures can exist on region.

see Resources, Structures, Moons