Mystara is the original default published campaign setting for Dungeons & Dragons. The actual original settings were "the lands around Castle Greyhawk," but Gary couldn't get his shit together in time for TSR going to press, and Dave's Blackmoor wasn't much more than a city and a map, so David Cook and Tom Moldvay expanded on the examples they used in the wilderness rules for D&D Expert Set and the starter module "X1: Isle of Dread." The working name for the setting was just "Known World," which you still might see on old maps.
TSR did the right thing with this world, by sketching out a variety of nations and cultures with a very broad brush (hippie elves over here, Corsican pirates over there, Arabs in here and some Cossack/Mongol horsemen up there...), and gave each nation to a writer to flesh it out into a sub-setting product. These products became the "Gazetteer" series, which even today are pure goddamn gold. Do not hesitate to buy it if you see it in an old bookshop, and download the fuck out of them if you catch 'em on
rapidshare Mediafire or Mega.
Arneson's Blackmoor was ret-conned into Mystara as an Atlantis-style mythic past, and the legendary source of lost magics, efreet bottles, secrets man wasn't meant to know, blah blah. Gygax was already pooh-poohing Basic D&D in favor of his "Advanced" version with mary-sue elves and strength penalties for women, so his "World of Greyhawk" was for AD&D only.
Mystara was discontinued by TSR (probably due to the brilliant marketing savvy of Lorraine Williams), but fans still keep it afloat. A dude in England made the titanic effort to record and correlate all the maps from splatbooks and Dragon magazine articles. There's even been projects to rewrite the settings for D&D 4E. In terms of 5E, there is a conversion project made by Mr. Welch, of "Things Mr. Welch Is No Longer Allowed to Do in an RPG" fame. He's actively trying to get the attention of the community, in hopes of opening the setting on DM's Guild.
Mystara was also the setting for the pure fucking awesome Capcom produced Dungeons and Dragons side-scroller beat 'em up games: Tower of Doom and Shadow Over Mystara.
This is a list of nations that were described in the Gazetteer series. They're all on a single continent of Mystara called "Brun," which is only 1/4 of the planet's total land area - and put together they only cover like 1/10 of Brun anyway.
- The Grand Duchy of Karameikos
This is your default middle-ages northeast Europe setting that you kinda expect with Dungeons & Dragons. Big coastline, crossroads for a few other nations, multiracial.
- The Emirates of Ylaruam
Arabian Nights all up in this bitch. A desert basin east of Karameikos, with a people simultaneously snooty with high culture and savage with scarce resources. Magic use is forbidden here, and wizards are hunted down and executed. Clerics are okay in their book, though.
- The Principalities of Glantri
Nation of magic-users, alchemists and militant atheists. Includes some variant magic rules to mary-sue-size your mage NPCs. Did I mention that being a cleric is punishable by death?
- The Kingdom of Ierendi
Fuck yeah pirates. They try to look like "respectable businessmen", so you get some mafia vibe here too. Includes some board-game kinda rules for ship-to-ship combat. Actually, if you have to pass on any of the Gazetteer series, you can pass on this one: Minrothad Guilds does a much better job.
- The Elves of Alfheim
You know the stereotype of elves being tree-worshipping hippies and a bunch of fucking snobs? Totally what's going on here. If it makes you feel better, they get buttraped by infighting and civil war in the canon history.
- The Dwarves of Rockhome
STRIKE THE EARTH! Included rules for characters that were both dwarves and clerics, and some AD&D conversion rules, 'cause AD&D is Gary's baby.
- The Northern Reaches
Fuck yeah Vikings. Included cardboard models of a Viking village for your minis, and rules for clerics to use rune magic.
- The Five Shires
hobbits halflings. You can be homebody big-bellied halflings, or the gypsy under-foot nuisance type.
- The Minrothad Guilds
Merchant of Venice time. Merchants on the high seas, including privateers for the cut-throat businessman.
- The Orcs of Thar
Need a place to lump together all the dirty smelly humanoid "monsters", so let's put them in and under this volcanic mountain range. Has rules for playing monster humanoids as player characters, has a hilarious booklet for what every orc infantry needs to know, and (!) includes a board game by Tom Wham.
- The Republic of Darokin
Fuck yeah musketeers. Includes rules for a new player class "Merchant", and retrofits trade routes into the previous gazetteers and Companion- & Master-level rules for commodity trading between nations.
- The Golden Khan of Ethengar
Fuck yeah Mongol hordes invading your shit. Adds a shaman player class.
- The Shadow Elves
Shaman player class here too if you missed the Mongol splatbook.
- The Athruaghin Clans
Here we have Amerindian-like clans, living mostly on an isolated plateau. Another shaman player class is given too, the totemic kind.
- Thyatis and Alphatia
As if all the stuff described before wasn't enough, there was also a boxed set for a couple of empires that covered more territory than all the above combined. Thyatis was built like Ancient Rome, and Alphatia is a another magicocracy but more industrial and bureaucratic than the cultish rulers of Glantri.
- The Hollow World
Oh shit, and then there was the entire OTHER campaign setting that was all Aztec/Inca-y that was supposed to be set inside the hollow planet of Mystara (just like our Earth where Hitler said the UFOs were coming from). It had dinosaurs, so it couldn't be all bad.
- No-Gaz. Nations
Several adventure modules expanded the description of Brun continent:
- X4&X5 added Sind and Hule to the west of "The Known World" map
- X6 added The Serpent Peninsula
- X9 went further west with "The Savage Coast" (later it even became the AD&D setting Red Steel)
- X11 added Wendar and Denagoth (north of Glantri)
- CM1 added Norwold just north of Known World.
Other products like Wrath Of The Immortals and Champions of Mystara, as The Poor Wizard's Almanacs gave a deeper insight on those lands.
- http://pandius.com - The "official" Mystara webpages
- http://mystara.thorf.co.uk - all of the maps of Mystara. ALL OF THEM