Humanity is the dominant sapient species on the planet Earth. In appearance, they do not differ from mainline (modern) humans, and demonstrate great variance in phenotype (skin tone, etc). However, the humans of this world very often are descended from non-humans in the distant past; humans who crossbred with fey, or jinn, or something else, but then the family line bred back with "plain" humans. Some scholars believe that the amount of nonhuman blood in a family tree is proportional to a given human's facility with magic; other sages decry this as racist balderdash, seeing magic as a skill which can be learned by anyone.
What is certain is that some humans who have unusual ancestry more recently in their family line (grandma was a dragon, for example) sometimes display an innate talent for sorcery, regardless of whether they are aware of their uncommon blood.