In short, Grimm takes a different angle on the severity and enjoyable presentation of a supernatural procedural (think Supernatural in its existence). Instead of a pair of monster hunters with a magic gun that are brothers, you have a man who is more or less a descendant of the legendary Brothers Grimm, armed with an extensive tome/book of listed monsters (known as Wesen or Wiesen, depending on its spelling in some languages) that works to police his home against the strangest creatures. It's very case-of-the-week in its presentation as always, with the occasional multi-episode arc. Overall, the show is rooted in using a lot of unknown actors that thrive well in their roles, so you never truly know who or what you're going to get (and that is a BIG sell in this show: the unexpected and the mystery). And after four seasons, it's still going pretty strong. We're already at five seasons in broadcast now, and with the show having eked out full season orders every time, this show is so close to syndication on other channels. If it does that, there's a very good chance it'll find more fans on other channels at whatever times deemed fit to air.
Overall, this show is for fans of modern supernatural or fantasy with a bit of police investigation/procedural...especially Supernatural fans.