The forest is home to a good many of the sentient inhuman species in England: centaurs, vampires, werewolves, nixies, dryads, faeries of all stripes, hags, phookas, and others. Many of the creatures here are more feral than one would find in the towns: most of the vampires and werewolves that roam the depths have gone somewhat mad, and live something like animals, consumed with bloodlust. Others have far more complex societies here than elsewhere; the centaurs are a good example of this. And of course, hundreds of non-sentient creatures, of varying levels of dangerousness, roam the depths. Some ghosts make their home here, too. No known human settlement exists within the forest, however.
There are also any number of rare and magical plants that grow in the forest; some in open clearings, needing the sun, others in among the roots of trees or the undersides of branches. Some of the flora here can only be seen at certain times, and others bloom at very specific intervals, or cannot be harvested without special tools or knowledge. Several professors at Hogwarts utilize the forest for this purpose, and are somewhat familiar with it, though no one is an expert. Hagrid was likely the most knowledgeable, but he wrote nothing down, and so what he knew was lost when he died.
Overall, the most important thing to remember about the Forbidden Forest is that it is unpredictable and dangerous. Students are disallowed from wandering there alone for a very good reason, and some of those who have flouted this rule have died or been lost.