Warforged are constructs built to fight in the Last War, and the humanoid races of Eberron regard warforged as unpleasant reminders of the brutality of that war. As they strive to be a part of society, warforged struggle to find ways to relate to the other races that created them.
Adventuring is one way that warforged can fit into the world, at least as well as any adventurer ever does. In the wilds of Xen'drik, the ancient continent of secrets, few people care whether you were born or made, as long as you can help keep your companions alive. A fairly large number of warforged choose an adventuring life to escape from the confines of a society that they didn't create, and at the same time engage in some meaningful activity.
Originally intended as nothing more than mindless war machines, the warforged acquired sentience due to the arcane experiments intended to improve these weapons of destruction. Through these experiments, the warforged moved away from being true constructs and became alive. The basic and most plentiful warforged are just slightly larger than humans. Larger varieties of warforged-like constructs can also be encountered, as these remnants of the Last War still exist.