The Forbidden Forest
"There are good things in there, too. The centaurs
are all right, and the unicorns..."
-- Harry Potter
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 (OP21) or detentions (PS15, DH15). Fred and George Weasley have spent quite a bit of time trying to get into it, naturally (PS8). The forbidden forest is sometimes referred to as the Forbidden Forest but often as just "the forest."
The forest is thick with trees -- beech (PS13), oak (PS15, CS15, OP28), pine (OP30), sycamore (CS15), and yew (OP21) are mentioned, as well as undergrowth such as knotgrass and thorns (OP30) -- 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 (which is what happened to Fluffy the giant three-headed dog ). Hagrid moves around the forest with respect but without fear. Any other person who has entered it, however, has emerged hoping never to enter it again.
There are areas around the edges of the forest that are part of the Hogwarts grounds but which are out of sight of the castle. Hagrid set up a paddock for hippogriffs in an area like this (PA6) and the dragons which had been brought in for the first task were confined here (GF19).
Hagrid's cabin is located at the edge of the forbidden forest.
During the Battle of Hogwarts, Voldemort used the Acromantula's lair in the forest as his base of operations. The Death Eaters drove out the Acromantulas and conscripted them to fight for the Dark Lord. It was into this clearing, still with webs draped overhead, that Harry walked to sacrifice himself (DH34, 36).
Acromantulas named Aragog and Mosag, together with their large family, lived in a dome-shaped nest in the very heart of the forest until Aragog's death during Harry's sixth year (HBP22). Hundreds of the giant spiders now dwell there, all decended from the original pair. Aragog was a pet of Hagrid's fifty years ago (CS15). The Acromantulas are among the most dangerous creatures in the forest (FB), although Grawp may have taken the title of 'most dangerous' away from them now.
At least 50 centaurs of fighting age live in the forest (OP33). In typical centaur fashion, most of them avoid humans and do not involve themselves in human activities, simply watching portents in the stars and uttering cryptic responses to questions.
Hagrid knows many of them by name and had a civil (though sometimes frustrating) working relationship with them until partway through Harry's fifth year. (Even in Harry's first year, he said that the centaurs weren't exactly friendly.) When Hagrid brought Grawp home with him late in 1995, the centaurs objected, and when Firenze accepted a job offer from Dumbledore, the centaurs turned against the staff and students of Hogwarts castle. See centaurs for more details about the Forbidden Forest herd (PS15, OP30).
a wild turquoise Ford
Here we have something that is not a living creature, but perhaps worthy of mention. The Ford Anglia was an automobile owned and enchanted by Arthur Weasley. After a bad fright and an exhausting trip across the length of Britain, the Ford Anglia escaped into the forest. There it became wild and took to trundling around among the trees (CS15).
Grawp the giant
When Hagrid and Madame Maxime acted as envoys to the giants in the summer and autumn of 1995, Hagrid learned of his mother's death and of the existence of a half-brother he'd never known: Grawp, a full-blooded giant. Since Hagrid has no other family, and Grawp (being a runt for a giant at only 16 feet) was being bullied, Hagrid managed to smuggle Grawp all the way back to the forest. This was the last straw for some of the forest's inhabitants, specifically the centaurs (OP30).
A herd of about 100 thestrals lives in the forest; Hagrid says proudly that he's probably the only person in Britain to have domesticated them (OP21). Grubbly-Plank says he's got them well-trained (OP17). Hagrid waited until Harry's fifth year to cover them in Care of Magical Creatures, considering them a special treat.
The unicorns are powerfully magical creatures living in the forest. They are extremely fast and are seldom hurt. Hagrid found one dead in the spring of 1992 and signs that another had been horribly wounded, and spent a night searching the forest for both the injured unicorn and whatever could have done such a terrible thing (PS15). In the ordinary way, the unicorns shed hair that becomes tangled in the undergrowth of the forest; Hagrid collects it to use as bindings on bandages for various injured animals because of its strength (HBP22).
The following creatures are alleged to live in the forbidden forest, but not on reliable testimony:
Many years ago, when Hagrid was a boy and a student at Hogwarts, he got in trouble a few times with the headmaster, Armando Dippet; Tom Riddle claimed that one reason for this was for sneaking off into the forest to wrestle trolls (CS17). We only have Tom Riddle's word for this, and he slandered Hagrid about "werewolf cubs" in the same sentence, so the alleged presence of trolls in the forest may merely be another flight of fancy on Riddle's part.)
Draco Malfoy said before his first year detention that he'd heard werewolves live in the forest, but Hagrid didn't affirm or deny this. When Harry subsequently asked whether a werewolf could have been killing the unicorns, Hagrid only answered the specific question asked, saying that a werewolf isn't fast enough to do that (PS15). (And after all, even if no werewolves live in the forest, a werewolf could have been getting in from the outside from time to time.)
According to Fantastic Beasts and Where to Find Them, there's only one type of werewolf: a human suffering from lycanthropy, like Lupin. Consequently, if werewolves do live in the forest, one wonders where they live the majority of the time, when they're normal humans.
Visits to the Forest
Harry has entered the forest eight times:
June 17, 1995 - Harry walked with Viktor Krum a little way into the forest when Barty Crouch senior walked out of it unexpectedly. Harry ran for help, but when he returned with Dumbledore, Crouch was gone and Krum lay unconscious among the trees (GF28).
mid-June 1996 - Hermione led Harry and Umbridge into the forest, making a lot of noise so as to attract the centaurs' attention; Umbridge got carried away by her prejudices, Grawp distracted the centaurs from Harry and Hermione; Ginny, Luna, Neville, and Ron followed them, and all six mounted up on thestrals to go after Sirius (OP33). Dumbledore said later that after checking on Sirius, Snape had searched the forest for Harry (OP37).
May, 1998 - Harry walks into the Forest, accompanied this time by his mother, his father, Sirus, and Lupin, to offer himself as a sacrifice so that Voldemort would be destroyed. As it transpires, the Killing Curse only kills the bit of Voldemort's soul inside Harry. Hagrid carries Harry's supposedly lifeless body out of the Forest back to the castle.
Walking very close together so that nobody would see them, they crossed the hall on tiptoe beneath the cloak, then walked down the stone front steps into the grounds. The sun was already sinking behind the Forbidden Forest, gilding the top branches of the trees.