Vistani
Though Ravenloft is primarily inspired by Gothic Horror, it owes just as much to the portrayal of that genre in Universal Horror and Hammer Horror films - particularly the latter. As such, when designing their setting, the writers at TSR felt that their setting also needed its own race of "morally ambiguous" magical gypsies to fit with the tradition.
Meet the Vistani, who were created to fill that niche. Foreign-looking humans who live as nomads, travelling the demiplane in caravans and making a living as entertainers, mystics and thieves, feared and hated by the sedentary people of the demiplane and looking down on the "Giorgio" right back in kind, smugly considering themselves superior to ordinary people and cursing them for their bigotry, even as they rob and manipulate those people whenever they see fit.
...Yeah, the Vistani do press a lot of 'Gypsy Stereotype' buttons. In their defense, they're not quite as bad as the Aperusa of Spelljammer, who're slathered with a lot more of the negative parts of that stereotype, but still, there are people who find them kind of embarrassing.
Vistani are distinguished from ordinary humans by not only possessing more knowledge of the dark creatures roaming the demiplane, but by natural affinities for magic and the ability to break some of the "rules" of the demiplanes. Vistani mystics can see the future via the power they call The Sight, an art normally impossible in the Demiplane of Dread. While technically both male and female Vistani are capable of The Sight, it is far more common among women- in no small part due to the fact that any male Vistana with the Sight is fated to become a Dukkar, who is destined to seek the destruction of the Vistani as a whole. Their caravans can reliably travel from domain to domain within it, even against the wills of the Darklords. Furthermore, all Vistani can use powerful magical curses, collectively known as the Evil Eye. There's also drawbacks to this, mostly in the form of needing to regularly travel, or else they will permanently lose all of their magical abilities.
Narratively, Vistani serve pretty much the same role that Elminster is supposed to serve in-universe: a convenient deus ex machina. Need to get the party somewhere? A Vistani caravan can take them there. Need to give your players a smack with a clue-by-four? A Vistani fortune-teller can do it for you. Party desperately needs some macguffin or another? A Vistani caravan will helpfully provide it - for a price. Need to explain why this latest monster or villain is being a nuisance? A Vistani curse went horribly right, as they seem to always do.
It goes without saying, but the combination of ethnic stereotyping and hamfisted plot device makes Vistani a rather contentious aspect of the setting for some fans.
In both 2nd and 3rd edition, Vistani were far too powerful to be used as a PC race, and so Half-Vistani were introduced instead; still with a dash of Vistani mysticism, but with some added "you are alienated from your peers" and "you were probably not conceived willingly" fluff.
4th edition's designers thought that this was stupid, and so they redesigned the Vistani into a collective culture of mystical, but not necessarily inhuman, plane-travelling nomads in Dragon Magazine #380. This meant you could finally play a full-fledged Vistani, something that was immediately met with skub.
The Vistani remained largely unchanged in 5th Edition's Curse of Strahd, but significant changes to their lore were mentioned in some fluff text in Tasha's Cauldron of Everything. One of the magic items in Tasha's, an artifact called Luba’s Tarokka of Souls, includes a sidebar about the Vistani and Mother Luba, the artifact's creator. It describes the Vistani as a racially diverse culture of travelers who wander the Shadowfell, and they often accept well-intentioned wayfarers into their bands. Mother Luba herself is stated to have been a Halfling Vistana. The fact that she still had some kind of power with her Tarokka deck despite not being born as a Vistana, as well as a line about how the Vistani have learned many secrets from the realms of the Shadowfell, implies that their abilities with divination and curses are things that Vistani have learned to do, rather than things they are born with. These changes were likely made to separate them more from the whole "Romani sterotype" thing, but to those who remember 4th edition, this basically looks like a recreation of the 4e lore for them.
Tasques & Tribes
Culturally, the Vistani are divided into three Tasques, which can be thought of as representing different nations or subraces of the Vistani people as a whole. Each Tasque is then broken up into a number of different Tribes, which are smaller subdivisions of the Tasques, usually revolving around distinct extended family lines.
Boem Tasque
Those Vistani who belong to the Boem Tasque are regarded as exemplifying the ideal of the Vistani as carefree wanderers. Their particular aptitude is an ability to temporarily charm and befriend others; whilst this near-supernatural charisma wears off once the Vistani leaves, the Boemians can reliably turn even the most hostile of initial greetings into a warm welcome. Notably Boemian tribes include the Naiat (traveling carnival performers) and the Corvara (masters-of-none, who tend to live down to the Vistani reputation as a bunch of lying, thieving, sneaky grey marketeers).
Kaldresh Tasque
Vistani of the Kaldresh are particularly known as skilled campfollowers, crafters, and traders. They particularly rely on trade, especially to the armies of other races, and Vistani of this Tasque seem to have a peculiar knack for not only homing in on places where people will need their mercantile skills, but for always having exactly what their future customers will need. Notable Kaldresh tribes include the Kamii (master smiths), the Equaar (master animal trainers and equestrians), and the Vatraska (expert healers).
Manusa Tasque
The Vistani of the Manusa Tasque are noted as naturally gifted with magic, even by the standards of the Vistani as a whole. The Canjars are all skilled wizards who always know what spell they need ahead of time and so can be prepared to cast it. One of their most famous member-tribes, the Zarovans, have a "non-linear" relationship with time, meaning they do not exist in the standard progression of past-present-future like other beings do.