Short answers, pulled from the story.
Lawrence Schick and Tom Moldvay began building the fantasy world known as the Known World in 1974 for their own Dungeons & Dragons game sessions. The name Mystara did not appear until September 1991 when Bruce Heard used it in Dragon magazine issue 173.
Mystara is a hollow planet with a habitable interior surface called the Hollow World that is lit by an eternal red sun at its center. This inner world serves as a cultural museum preserving societies extinct on the outer surface while two huge holes at the poles allow passage between the worlds through cold unlit stormy anti-magic areas.
The eastern portion of Brun includes the Thyatian Empire which resembles the Byzantine Empire, the Grand Duchy of Karameikos containing Threshold, and the Principalities of Glantri ruled by wizard-princes. Other regions include the Ethengar Khanate functioning as a Mongol-like society, the Republic of Darokin inspired by medieval Italian mercantile states, and the Emirates of Ylaruam.
The Savage Coast area spans approximately 2000 miles west of the Known World and is affected by the Red Curse which kills inhabitants through mutation unless they wear fictional metal cinnabryl. Specifics of this sinister enchantment include mutilation of the body and extreme degeneration of physical and mental health that imprison people within the cursed area if they leave it.
Mystara features ascended immortal beings called Immortals who govern the fate of the world through specialized rules rather than traditional deities found in other fantasy role-playing game settings. Only Immortals inhabit Matera where they live in a city named Pandius to meet and watch over Mystara while advancing player characters into ranks exploring the multiverse system.