Basically a sub human species. They look and act like humans but amongst their own kind once provoked they vogue (morph) into a human with predominate animal like traits. Some will vogue and have a wolf like countenance, great smell, night vision, ect. Others reptilian, others fowl like, others magical powers, and some are like creatures from mid evil stories. A Grimm is one who's countenance doesn't change, they are supreme fighters who have developed skills and specialized weapons to combat the other creatures. That was all mid evil time period. In todays world they all are more humane acting. Of course some go astray and that's where the Grimm steps in. In the mid evil times the Grimms took it upon themselves to destroy all the others. Only when the others vogue can they identify a Grimm. This particular Grimm is also a detective with the Portland police department. It has been a very interesting show but I could tell in the last 5 or 10 episodes they were probably going to end it soon. In the same manner of Jericho, it was real good then it started going astray like they were in too big of a hurry to do a good job to put a show together.