Hmmmm..... Good question.
I always thought of dark fantasy as fantasy that dealt with tropes or themes more commonly seen in horror--creatures like vampires, possession etc., handled in a more "scary' way, maybe, than in UF or paranormal romance. And maybe the tone is more gothic or literary (though there's gothic fantasy as a category too).
I think of Grimdark as being an offshoot of more "traditional" fantasy set in secondary, often medieval or renaissance like worlds, but deconstructing the usual uplifting or optimistic "knights in shining armor" tropes to have a more cynical (some would say "realistic," but that is arguable) take on things with a very high body count. Or as one article I read once said, "knights who say the f word a lot."
The protagonists in grimdark tend to become deeply flawed. It can be vaguely satirical (like Abercrombie's First Law books) or more serious in tone, with a character arc that plunges the protagonist on a very evil path (like Lawrence's
Prince of Thorns). Most characters suffer horribly, but the characters who are "good" or closest to it tend to suffer the most, and are often more likely to die or to suffer in ways that their goodness and optimism don't survive.
But the differences are hard to pin down, and I see the terms being used interchangeably or in overlapping ways. Maybe the best way to parse the difference is to google "top grimdark novels/authors" and "top dark fantasy novels/authors." There will be some overlap, but there will be people who are only on one of the two lists.
I think you may also have to be male (and I'm only half kidding here, as female authors, even those who write pretty dark, cynical stuff, like Anne Bishop and Kameron Hurley,
never make the lists) to be considered a Grimdark writer. Some of the people referred to as Dark Fantasy writers (like Gertrude Barrows Bennett) are women.
I think "Grimdark" is more recent as well (originating as a term in the Warhammer games) while the term "dark fantasy" has been in use far longer. There are older fantasy novels (like the Elric series, Leiber's stuff, Karl Edward Wagner's books, and Gertrude Barrows Bennet and Melanie Rawn's work) that deconstruct or satirize familiar fantasy tropes and follow a "fallen" or less virtuous arc with their protagonists, but they don't tend to be categorized with Grimdark.
Thinking on it, maybe the main thing between Grimdark and traditional dark (also satirical) fantasy is the degree of graphicness or explicitness. There are torture scenes in the Elric books, for instance, and characters have been having sex in sword and sorcery style fantasy for forever, but the scenes in the First Law series are much more graphic. Also, there's more profanity in modern grimdark, and the voice tends to be more modern or contemporary in tone.
This sounds like I'm ripping on Grimdark, but actually I do like some of the books that are labeled as such. I liked the first three ASoIaF books, and I loved some of Abercrombie's books. But I never understood why people often imply that they
invented fantasy with a cynical or satirical or deconstructive take on traditional tropes.