I have played fairly little Dungeons and Dragons before the fifth edition, and have some gaps in my knowledge of the lore. One of the bigger holes concerns the nature and differences of the courts of Seelie Fey and Unseelie Fey.
The Dungeon Master's Guide has an info box concerning the courts of Feywild, but it does fairly little to describe the nature of the courts:
Dungeon Master's Guide, page 49 (Seelie and Unseelie Fey)
… Seelie and unseelie do not directly correlate with good and evil, though many mortals make that equation. Many seelie fey are good, and many unseelie are evil, but their opposition to each other stems from their queens' jealous rivalry, not abstract moral concerns.
Rather, it seems to mostly point out that the two courts are not good and evil, but otherwise leaves me guessing about their differences. However, if many mortals do generalize them as such, it hints at the Unseelie indeed being a bit more unscrupulous in at least some ways.
A bit of online searching brought up an old Wizards of the Coast Article, Seelie and Unseelie Courts. Some excerpts on their description of the Unseelie court:
Unlike the selective, restrictive Seelie Court, the Unseelie Court welcomes anyone and everything with even a drop of ancestral fey blood. Fey can and do breed with anything, creating odd, mixed creatures. Most species consider the offspring grotesque monsters. The mutant creatures gravitate towards the Unseelie Court, which welcomes them and gives them an environment where peculiar physiologies and abilities are the norm.
…
After a millennia of indiscriminate breeding, the physical appearance of the Unseelie Court mirrors the macabre. Twisted columns, trees forced into unnatural growth by royal gardeners, are scattered haphazardly through the hall. Curtains of shadows hide blood-soaked alcoves.
However, I find this information, written for earlier editions, to be in direct contradiction with 5e source material:
Dungeon Master's Guide, page 49 (Seelie and Unseelie Fey)
Ugly denizens of Feywild, such as fomorians and hags, are almost never members of either court …
Volo's Guide to Monsters, page 52 (Hags, Dark Sisterhood)
Because both the Seelie Court and the Unseelie Court appreciate and revere true beauty among the fey, hags are almost never found in either place. The Summer Queen and the Queen of Air and Darkness recognize that hags have valuable knowledge and impressive magic, but they can't abide the stain on the beauty of their surroundings, so most hags are excluded from both courts.
So, clearly Unseelie Fey are no longer supposed to be revelers in the grotesque. What, then, are the differences between the two Fey courts in DnD 5e?
Best Answer
D&D Sources
I think Weckar E. is basically correct in that this confusion stems from the fact that, like many elements of D&D, the fey courts are cobbled together from a mishmash of not-terribly-consistent folklore. There seem to be two competing views:
It sounds like more sources tend toward the first view, but I think there's enough material provided for you to choose whichever one you feel is right for your game.
Other Sources
Other depictions of the two courts that I'm familiar with tend more toward the first view. Often fey from both courts are equally dangerous in their own ways - they merely express their unpredictable personalities and the power of nature differently.
The Dresden Files novels, for example, play up the Summer and Winter connections. The Seelie are bright, colorful, and generally pleasant enough on the surface, but they can also be quick to anger and terrible in their power, like a summer's day giving way to a powerful thunderstorm. Alternatively, they might simply get carried away or be otherwise heedless of the consequences of their actions, turning from pleasant warmth to a dangerous heat wave.
The Unseelie are cold and vicious, but aren't entirely without compassion. After all, winter still has its mild days; it is not an unrelenting blizzard. They still experience some form of love, or at least emotional attachment, and while they might see mortals as playthings most of the time, they may develop a certain admiration for mortals who prove themselves capable of keeping up. Even then, mortals must remain on their guard - it's not uncommon for the Unseelie to have a sadomasochistic streak, and their version of kindness might not seem so pleasant to others.