From Call of War Wiki
There are 5 terrain types:
Motorized units and tanks are faster on plains and can use their maneuverability to inflict more damage to their opponents.
|Forest and Hills
Uneven terrain in which ordnance and ambush units can inflict more damage to their opponents. Motorized units and tanks move slower in forest and hills. This terrain can either appear as forest or as hills on the map, but is combined in the interface.
Rough mountains reduce the movement speed of all units. Certain ordnance and infantry units can inflict more damage to their opponents here.
Urban areas in which Infantry and certain counter units inflict more damage to their opponents.
Only ships can travel and fight effectively on sea. Land units which are sent to sea will automatically be converted to a transport ship, which reduces their fighting strength and survivability.
The terrain of a province is indicated by the province texture on the map and by an icon in the province bar after selecting the province:
The effects of the terrain on the speed and damage statistics of a unit type can be seen in the unit details panel. The unit details panel can be opened by clicking on a unit portrait after selecting an army or by clicking on the info button in the unit production menu.
Try to avoid fights on terrain where your enemy’s troops have the advantage and try to lure your enemies into fights on terrain where your own troops have the upper hand.