Grimm, or the Creatures of Grimm, are the universal antagonists of RWBY, inhabiting various parts of Remnant.
They are described as "creatures of destruction" and lack a soul; hence, they are unable to use Aura. They are also drawn to feelings of negativity- such as envy, sadness, loneliness, and hatred- often congregating towards the source of these emotions.
At one point, ancient cultures believed that Grimm were animals possessed by evil spirits or were the spirits of once tortured animals.
Grimm Species
http://rwby.wikia.com/wiki/Grimm