The many races of Eberron exist side-by-side in all aspects of life. The majority of people of all races in Khorvaire think of themselves as citizens of their nation first, and members of their race second.
Humans originated on the continent of Sarlona, which is today ruled by a secretive and insular empire which all but forbids outsiders. It was a human dynasty that ruled Galifar, and descendants of that family still rule Aundair, Breland, and Karrnath today. Shifters and changelings are generally believed to be offshoots of the human race, their ancestors having mixed their bloodlines with lycanthropes and doppelgangers respectively.
Most half-elves are actually descended from purely half-elven families whose human and elven ancestors came together centuries ago; most half-orcs hail from communities where humans and orcs live together (most commonly in the Shadow Marches) though several bloodlines entirely consisting of half-orcs for generations can be found, especially in House Tharashk.
Elves can trace their origins back to the island continent of Aerenal (and even further to slavery in the giant empires of Xen’drik), but most Khorvairian elves are descended from historical waves of immigrants. The Tairnadal elves of Valenar are an exception, having come over during the Last War to fight as mercenaries but then staking a claim to their nation on the grounds of their ancestors’ conquests long before humanity came to Khorvaire.
Dwarves have existed in Khorvaire for as long as history records, but the native dwarven society of the Mror Holds has only recently elevated itself from barbaric tribal conflicts. Gnomes have lived in the region now occupied by the gnomish nation of Zilargo for an equally long time, though scholars believe they originated from the fey realm of Thelanis. Likewise halfling clans have roamed the Talenta Plains for as long as history records, but their migration into the more civilised lands is fairly recent; nomadic clan customs still often prevail even in the largest cities.
The youngest race in the world is the warforged. All were constructed in the creation forges of House Cannith to fight as soldiers in the Last War; the Treaty of Thronehold which formally ended the war declared all warforged free citizens of the nations for which they fought, and outlawed the construction or use of creation forges. Most warforged find it difficult to integrate into the normal way of life of their nations; some heed the stories of the Lord of Blades, a prophet or visionary who is said to be building a nation of the warforged within the Mournland.