Genasi were planetouched humans infused with the power of the elements. Along with Aasimar and Tieflings, Genasi were the most common planetouched found on the world of Toril.
Technically speaking, the Genasi were not a race but rather a general classification of humans who had a heritage (usually unknown) that included some planar being from one of the elemental planes, most often a genie, which is whence their name is derived.