From the Forgotten Realms Campaign Setting book:
Faerun is home to hundreds of intelligent creatures, ranging from the teeming kingdoms of humankind to the secret fastnesses of terrible creatures whose entire species numbers a score or less. Like humans, these peoples run the gamut from grotesque to beautiful, from murderous to beatific.
The great story of Faerun is, in many ways, that of the rise of humankind and the fading of the ancient empires of those who came before. Over thousands of years, humans have brought an end to the old ways. Elven cities lie in ruins, abandoned to human encroachment. Hills and dells once the homes and hunting grounds of goblins and giants are now dotted with human fields and pastures.
Human pride and folly have brought untold disaster down on Faerun more than once, and the ever-growing lands of humans encroach on the territories of older races both benign and fierce. The fundamental questions are clear: Can the old races survive the dominance of humankind? Or will humans overreach themselves, as they have so many times before, and bring down upon all a dark age of unimaginable horror?
Of the many races of Toril, a dozen or so account for nine-tenths of all the folk who live in the world today. Humans are the most numerous. They are a race of kingdom-builders, merchants, wizards and clerics whose crowded cities lie scattered across the face of the continent. Young and vigorous in comparison to the other races, humans hold the future of Faerun in their hands – for good or for ill.
While humans were still eking out a subsistence in scattered, disorganized bands, two older races – dwarves and elves – raised mighty realms in the mountains and forests. The zenith of both races in now past, but Faerun is filled with wonders of stone, wood, and magic they wrought at the heights of their power. Grim dwarven citadels filled with the clamor of industry and breathtaking elven cities as graceful as spun glass still stand, even as year by year human dominion grows.
Though they never commanded the power of the dwarves or the elves, halflings and gnomes have adapted better to the rise of humankind. Halflings have prospered, taking advantage of the situations created by cultural conflicts between the humans and the elder races. Although halflings hold lands exclusively for their people in only a handful of places, their settlements can be found throughout most human lands. Gnomes prefer more reclusive dwellings and do not raise mighty cities, but, like halflings, their homes and settlements are scattered through a dozen human lands.
Other races are sometimes considered civilized folk, too, despite their smaller numbers. Centaurs and fey roam the great northern forests, good of heart but growing ever wary of human incursions. Merfolk rule vast underwater domains in the warm seas of the south. Proud wemics roam the endless plains of the Shaar. But their numbers are few compared to even small human land.
Against the young human lands and the ancient refuges of the older races stand ranged a great number of enemies. Foremost among these are the savage peoples – goblins, orcs, ogres, and all their kin. Breeding fierce warriors in the dark mountain fortresses and noisome cavern dens, they regularly burst forth from their strongholds to pillage and slaughter villages and towns unfortunate enough to lie in their path.
Faerun is home to creatures far more malevolent, cruel, and calculating than orc chieftains and rampaging ogres. The deeps of the Underdark house sinister and powerful beings such as the drow, the beholders, and the mind flayers. These terrible creatures dream of enslaving the surface lands and feasting on human cattle while they rule as the overlords of all Toril.
Neither the uncounted hordes of goblinkind nor the dark powers that lie beneath the surface world are the most dangerous threat to human cities and realms, however. That honor must be reserved for the most terrible and awesome creatures of Faerun – the dragons. No one knows just how many dragons soar through the icy spires of the Spine of the World or slither through the depths of the Forest of Wyrms, but even a single dragon can spell doom for a city. From time to time, great numbers of dragons take flight at once and wing across the face of Faerun in a terrifying rage, burning and devouring at will.