To the east of Hogwarts castle is a large, dark forest. This forest is strictly off limits to Hogwarts students, except in the course of occasional Care of Magical Creatures lessons or detentions.
The forest is thick with trees as well as undergrowth such but there are paths, brooks, and some clearings. Off the path, the way is almost impassable, although people have moved through it and creatures as large as Acromantulas and a Ford Anglia have managed to get around.
The forbidden forest is home to a wide and strange assortment of creatures. Anything wild and dangerous that needs a place to live is put there.