The Empire is mostly populated by Humans. There are lots of human cultures, from the nomadic Woad, the Edene that were one of the first real colonisers of Rûl, the men of Gog that led to the Witch-King’s empire of Gog, the Kalasans that arrived and formed a coalition to topple it and create the current Empire, and many more.
Before the humans, there were the fey, magical creatures of lots of different forms and capacities and with inscrutable minds. Few can be found, as most fairies are now retired from the world into their own Hidden Worlds, but some stayed behind. Apart from those hidden in forests and other areas, Goblins are fey that live with normal mortals (for most other fey are said to be immortal) and tend to take care of menial tasks no one else cares for. They have thus, integrated well into human society, although just in the outskirts of society. They all look different, but are normally easily recognisable with their small twisted forms. Fauns, the next most easily found although much rarer than Goblins, are not really accepted, basically anywhere. They are said to be the offspring of human and fey, and have goat legs and horns to show for it.
Dwarfs mostly live in their kingdoms in the mountains, digging for riches, but some do travel and mingle a bit with humans, if only for commerce (although of course sometimes for some other reason). Halflings appeared one day in the Empire, coming from the Endless Steppes. Being very close to humans, very sociable and considered lucky they integrated very well into the Empire.
The Endless Steppes are mostly inhabited by Centaur tribes which you don’t want to mess a lot with. Men tried several times to conquer their lands, but ultimately failed.
The world is a place of magic - where it is present in many ways and magic casters of different kinds are not rare. But technology has been advancing a lot the last centuries, and it mixes well with magic when it must. Some time ago, some engineer found the way of building animate constructs that housed souls, and thus could have autonomy and a sense of person-hood. Clockworks roam the lands (well, most of them in bigger cities) since.
The Jotun are huge humanoids that lived mostly in the deep south, although they were said to live also in other places like the Northern Reach. They have been in a long on and off war with the Empire, which used magic to create the Orcs from them. Orcs where used for heavy work and as good soldiers and were slaves to the Empire until very recently. Also mighty - run if you see one - Trolls live mostly under ground, usually in mountains. They say they can’t see the sunlight, but I’m not willing to test it.
Lots of other creatures exists - some natural (the lizardmen for example), others magical (elementals), but most of these you don’t see in polite society. Undead are known to exist, and is very rude to bring them to any party unannounced. Some people talk of Dragons, Demons, Devils… but few people that have seen them live to tell the story.