Just as there is a sliding scale of hardness ranging from fantasy (less real/hard) to science (more real/hard), so in SFF and indeed in fiction in general there is a similar scale of bleakness, which ranges from zero bleakness with happy escapism (less real) to full on bleakness with grimdark (more real). All of which refers to the particular ambience of the story, reflecting the moral condition of the world it describes, and of its protagonists and antagonists.
Actually this sliding scale is still too simplistic because sometimes a story may be escapist in some ways and grimdark in others. This is because for the sake of simplicity three different variables are considered here together: plot armour (how indestructible is the protagonist and his or her friends and companions), bleakness of universe (how much natural justice is there), and morality of the protagonist (is he or she a moral; person or basically a sociopath).
This gives the following tentative and provisional list:
Fluff: Fluff, or Escapism, is just that, a feel good story to escape from the bleakness of the world. The morality is sometimes simply be white on grey where no one is really evil. And even when the villain is evil, they and their minions aren’t harmed, although they do end up spending a long time in jail for their misdeeds. Examples include: kid’s cartoons, swashbuckling, pre-80s superhero comics, etc.
Adventure has a bit more gravitas, in that generally an important character, like a mentor, friend, or love interest, will generally be killed in the course of the story. This is to emphasise how evil the bad guys are, and give the main character extra motivation to fight the villain more strongly and defeat him. Morality is universally white hats vs black hats, with no moral nuances. Despite the various twists and turns and dramatic tension of the story, in the end not only does the hero escapes unscathed, but even the hero’s companions and loved ones, thanks to their powerful plot armour Morality is still white hat vs black hat. Sometimes even villains are converted to the light, although irredeemably evil villains may be killed, in order to illustrate the fairness and goodness of the universe. Examples include Star Wars, Lord of the Rings, action movies, a lot of supernatural romance (the Twilight books being the most extreme example of “no one dies” apart from some redshirt humans eaten by vampires) and TV series like Star Trek, Buffy, etc. Nevertheless, despite all the twists and turns, the main protagonist is never in any danger, and the reader or viewer knows that the good guys will prevail (even if they tragically lose a few companions). Also included here is optimistic or Utopian science fiction, for example on the left the Culture of Iain M Banks, and on the right Heinlein’s Starship Troopers (and most right wing military SF in general)
The Flawed World. In contrast to the morally comforting fantasy of Escapism and Adventure, this is a more realistic scenario, which we see in politics and society every day, in which good people contend against a rotten system and corrupt power brokers and vested interests. That’s not to say there can’t be a happy ending. More gritty military science fiction, such as Halderman’s Forever War, would be included here.
Gritty or Punk is all about the anarchist and dystopian, and sometimes the criminal, underbelly of society, in which heroes aren’t necessarily good, even if the villains are usually bad. The morality is grey vs black, the hero is an antihero (as in “Han shot first”) who gets by in an amoral world., usually through acting selfishly. Examples are Noir fiction, and the various punk genres like cyberpunk and spacepunk can be included here.
Crapsack (or Mild Grimdark) is like Gritty / Punk but worse. The name is from TV Tropes. and describes a world is basically terrible in every way. In fact the dystopia of the world is almost comical. Crapsack is like Grimdark but without the extreme despair. Judge Dredd‘s Megacity One is a good example of a Crapsack universe. So is The Matrix, with its gnostic despair of matterr and the machine. The Star Wars movie Rogue One can also be considered in this category, as its ambience is very different to the nostalgic The Force Awakens. Here the noble morality of the protagonists (white and black hats) contrasts with the bleak ending, and the fact that everyone is an expendable dust mote in the larger star wars universe which follows the happy Adventures of the Skywalker family (where even Darth Vader, are redeemed in the end.)
Medium Grimdark presents a bleak and amoral universe inhabited by a tortured hero or antihero, who sometimes is forced to do, or who has done, very un-heroic/un-antiheroic things, things that he or she is not proud about. Moreover, the Grimdark protagonist may continue to do these things. Also, some of his or her friends or companions die, often in horrible ways, because even if the mooks are incompetent, the main villains are not. The morality is now grey vs grey, with no good guys but conversely no real villains. The classic examples of Medium Grimdark is Warhammer 40,000, the franchise that created the term “grimdark”, and 1980s and post 80s comic heroes like Frank Miller’s The Dark Knight.
True Grimdark is distinguished from Gritty, Crapsack, and Medium Grimdark in that no one is redeemed and almost everyone dies. In other words, there is no Plot Armour for anyone. Morality is grey vs grey, or even occasionally black on black. The classic example of True Grimdark, and indeed of Grimdark in general, is George R R Martin’s Song of Fire and Ice series (HBO Game of Thrones). There is however a difference between Grimdark and Horror. Horror just kills for the sake of it, there is no story, or if there is it is one where the protagonist is assured of having a horrible end. Dark Fantasy therefore is just the flip side of fluff and escapism, as both are equally lacking in realism. What makes True Grimdark so potent is that nothing is certain either way. If there is no Plot Armour, neither is there Horror’s plot anti-armour.
Some other sliding scales
The following are from the excellent TV Tropes website. In no order but alphabetical: