Mystara was first known as simply The Known World in early editions of DnD and first mentioned in the Module X1, Isle of Dread. Mystara consists of the continent of Brun, the continent of Skothar, and the continent of Davania, plus the island continent of Alphatia. In the officially published DnD material, the Known World concentrated on the eastern portion of Brun along with the lands of the Sea of Dawn. The continents of Mystara supposedly resemble those of the earth, making it a little easier for players and dungeonmasters to draw on information from history to create stories. Yes, you have it right, Mystara has the opportunities to teach players of DnD about history, culture, and other educational topics. Nifty, huh?
Mystara has a level of technological development that resemble the 15th century Europe, but without gunpowder. Nations of the known world display a great range of government types. Some nations are populated entirely by demihumans and humanoids. It includes the Thyatian Empire, which could be compared to Byzantine Empire; the Grand Duchy of Karameikos (which includes the town of Threshold, the default setting of many classic D&D adventures from the various boxed sets), comparable to medieval southeastern Europe; the Principalities of Glantri, which is similar to medieval western Europe, ruled by wizard-princes; the Ethengar Khanate, a Mongol-like society; the merchant-run Republic of Darokin, which is based somewhat loosely on the mercantile states of Medieval Italy; the Emirates of Ylaruam which have an Arabic flavor; the Heldannic Territories, ruled by an order of religious Knights devoted to the Immortal Vanya, similar to the Teutonic Knights; the Atruaghin Clans, which have an Amerindian feel; the nation of Sind, based on India during the rule of the Mughals; the Northern Reaches Kingdoms of Ostland, Vestland, the Soderfjord Jarldoms, based on Scandinavian kingdoms at various periods of history; the Dwarven nation of Rockhome; the elven Kingdom of Alfheim; the Halfling lands of the Five Shires;and the Alphatian Empire, ruled by wizards and other spellcasters.
Mystara is also the setting of Hollow World. Mystara is a hollow planet, with a habitable surface on its interior. This world is lit by an eternal red sun at the center of Mystara, and serves as a “cultural museum,” preserving the societies that have become extinct in the outer world. The existence of the Hollow World is not, in general, known to the inhabitants of the outer world. The poles are actually huge, subtly curving holes that allow passage between the outer and inner world, although it is a long, hard trek through a cold, unlit, stormy and anti-magic area. The curvature of the holes is so subtle that explorers from either surface do not notice the transition until after it is already made, causing quite a shock for most.
Lastly, it is also the settings for the original Blackmoor by Dave Arneson and Greyhawk by Gary Gygax. Mystara (like Greyhawk) also incorporated the Blackmoor setting by placing it in the world’s distant past. Blackmoor evolved from a feudal kingdom into a highly advanced civilization, using more and more powerful—and destructive—technology using steampunk elements. It ended itself in an apocalyptic explosion so devastating that it changed the climate and geography of the planet as a whole.