From Bulbapedia, the community-driven Pokémon encyclopedia.
Forests are secluded, wooded areas that can be found on Earth and in the Pokémon world. They tend to have Grass-, Flying- and Bug-type Pokémon, such as Caterpie although Pokémon based on animals that live in the forest, such as Pikachu may be found in them. In total there are 100 known forests in the Pokémon world. In each main series region, there is at least one forest, with major forests including Viridian Forest in Kanto, Ilex Forest in Johto, Petalburg Woods in Hoenn, Relic Forest in Orre, Lyra Forest in Fiore, Eterna Forest in Sinnoh, Pinwheel Forest in Unova, and Vien Forest in Almia. In Pokémon Black and White, the first version-exclusive forest, known as White Forest, was shown.
Forests are in almost every region of the Pokémon world, although they are not as common as caves. In the main series Pokémon games, they are mostly mazes which usually have at least one tree that Cut can be used on. Besides the aforementioned Cut trees, there are many other different types of trees that can be found in forests, such as one type which can be Headbutted, which results in Pokémon, such as Spinarak, appearing. Others can have Honey slathered on them, which attracts Pokémon, such as Munchlax, to them. The final two types are found in Johto and Sinnoh-based games respectively.
List of forests in the Pokémon world