Downworlders are hybrid creatures. It is the widely used term to refer to beings that are said to be part human and part demon, even said to be the demonic counterpart of Nephilim, who are part human and part Angel.
Downworlders include the Children of the Moon (Werewolves), the Children of the Night (Vampires), the Children of Lilith (Warlocks) and the Fair Folk (Faeries). Also mentioned are peris, ifrits, nixies, and mermaids. Although once mentioned by Jace, it was never made clear if zombies really counted as Downworlders or if this was a joke.
Every fifteen years, the Accords are signed between the Downworlders and the Clave, the Nephilim. According to the Accords, as long as the rules stated here are not broken, they can live in peace without bother and may even provide each other help when the need ever rises. Also, each of the four races has been accorded a seat on the Nephilim council as of the third installment in the Mortal Instruments series, City of Glass.