A forest has a Line of Sight Sight value of 5. Models in or behind forests cannot see or be seen through it if there is more than 5" of forest between them and the enemy. If it is less than 5", they can see and be seen through it for all purposes, as long as you can draw a Line of Sight to them as normal. The 'base' of the forest is otherwise not considered to block line of sight.
Models with the Fly special rule that begin or end a flying move in a forest, must take a Dangerous Terrain test.
A unit in a forest counts as being in soft cover provided that the majority of its models are within the forest. Similarly, if a model in a shooting unit has to trace its line of fire through a forest (because it's shooting at a target beyond the forest), then the target benefits from soft cover. This applies even if during the game you rearranged a forest's trees around a unit, or even removed them – just imagine the trees are still in their original position. This penalty does not apply if the shooting model is drawing a line of sight out of a forest it is in.
It is difficult to form tight ranks when fighting in a forest. a unit with the majority of its models fighting in a forest can therefore never gain rank bonus.
Below is a selection of different forests that you can choose to include in your games: