Well, what do you want to know? *laughs*
Let me try and summarize.
The nomer "Known World" arose from the fact that the information presented on it in Isle of Dread was very brief. The idea was the DM would make up their own name for the world at large. It started out very small (compared to places like Forgotten Realms) with lots of varied cultures and environments packed close together. Later on adventures pushed the boundaries a bit north and west, bringing in Norworld and Wendar and hinting around about the Savage Coast. Eventually they did a Hollow World expansion that dealt with the world inside Mystara, and then the Princess Ark series in Dragon expanded further west into the Savage Coast. AD&D didn't really push it much further, going a bit west into the Orc's Head Penninsula and Nimmur (plus a brief bit on the Arm of the Immortals).
The best source of Known World/Mystara information is the Gazateer series (GAZ for short), the Hollow World Boxed set, the the three Hollow World gazateers. If you don't mind the changes wrought on the setting from the Wrath of the Immortals setting (Alphatia sank, Alfheim got taken over by shadow elves, a comet took out part of Glantri), the Poor Wizard's Alamanacs are great resources too.
Some quick bits about nations:
Karameikos: "Baseline" nation where campaigns are assumed to start. It was a spooky old woods until Thyatis took it over, then a minor noble "bought" it and renamed it. Because of that, half the population is Thyatian, half is Traladaran (a slavic race). It's very standard D&D middle ages European in flavor. Bargle lives here. During the Wrath of the Immortals, the nation changed from the Grand Duchy under Thyatian control to a straight up Kingdom.
Thyatis: Dominant nation of the mainland, largely Roman in flavor. They have an emperor and a senate and corruption is pretty rampant. Thyatis controls a number of islands just of the coast of the mainland and has a stake in the Hinterlands across the Dread Sea to the south. They're half of the Dawn of the Emperors boxed set.
Ylaruam: A desert nation north of Thyatis, arabic in nature similar to the moors. They fanatically worship the immortal Al-Kalim who speaks of a day when the desert will bloom again. There are two factions - the Preceptors who are more progressive and open and the Kin who are really hard-line fundamentalists that hate infidels, magic-users, and elves.
Rockhome: A mountainous region that is home to the dwarves. Most of them live underground. The dwarves are generally friendly to other races, but they are typically hard to get along with. Dwarves hate shadow elves with a passion, and the fact that other surface nations don't share the same hatred typically strains relations.
Darokin: A mercantile nation where gold rules and the top-earning merchant houses make most of the decisions. It has a victorian Italy feel to it, as it's more of a rapier and breastplate culture than two-handed sword and full plate. Most of the grain in the Known World is grown here. It borders most of the nations as well (and completely engulfs Alfheim).
Alfheim: Home of most continental elves (the sylvan sort, otherwise known as 'high' elves in 2e or just plain elves in 3e). It was a featureless plain until the elves led by Maelidean Starwatcher traveled from the Sylvan Realm and used their magic to transform it into a forest. Because it's unnatural, there are lots of good and bad magic points that cause problems and the weather is very routine as to when it rains and whatnot. During the Wrath of the Immortals, the shadow elves corrupted the forest and drove out the elves, renaming it Aengmor. Of course, they suddenly realized, "Dern, we're killing this place we fought so hard to win."
The Five Shires: Think of The Shire from Lord of the Rings and you're halfway home. This is where the halflings have lived and fought, having been slaves of humanoids more than once in the past. The nation is ruled by a council of five sheriffs, hence the term "Five Shires." It's next to Karameikos and has lots of problems with the Black Eagle Barony (and it's baron, Ludwig Von Hendriks) along the border.
The Broken Lands: A blighted wasteland caused by, IIRC, a Blackmoor device that the shadow elves blew up. I'm not 100% sure on this because sources tell different stories. Lots of humanoids live here, and Thar is a major player in the area before the Wrath of the Immortals. Shadow elves also have a bit of a stake here, because their ancient city of Aengmor is buried beneath it.
Glantri: A magocracy ruled by a council of wizards. It's a really chaotic place with lots of intrigue (and even more so once you get past the Wrath of the Immortals). Some of the cultures you can find there are Germanic (Prince Jaggar's land), Translyvanian (the vampire Morphail's), and pseudo-Jewish (the Flaems). There are two elven principalities, a Scottish one ruled by a lich, and more. It sort of fluctuates because some are ruined during the Wrath of the Immortals. After Wrath, one is ruled by a Night dragon (an evil undead dragon), one by a werewolf that is surprisingly heroic, and one by a kobold (or mutated shadow elf that thinks he's a kobold, depending on whether you are in OD&D or AD&D). Glantri City is like Venice in that its full of canals, and the Great School of Magic is THE school of wizardry on the mainland.
Ethengar: Ruled by a Khan, this is a mongolian culture on a vast steppe. Except for a mountain in the middle that is a place of evil magic, it's largely a land of horsemen who absolutely hate Glantri and raid them constantly. They are a shamanistic people, and the Ethengar GAZ has an OD&D shaman class in it.
Northern Reaches: This is a nordic, viking culture full of trolls and runes and crafty dwarves that are alchemists. There are actually three nations here, Ostland, Vestland, and the Soderfjord Jarldoms, and each represents a different type of era of the viking people.
Heldann: Depending on if you're playing in the Gazateer era, or right around/before the Wrath of the Immortals, this is a different place. Originally it was a generally lawless democracy of free farmers. However, the Knights of Vanya, a militaristic (and Germanic influenced) group of Thyatians conquered the region in the name of their patron. Vanya demands conquest, and the Knights started spreading their influence around the Known World. They are constantly at war with Alphatia.
Alphatia: The other magocracy in the Known World, Alphatia is an island to the east of the mainland ruled by an Empress that uses magic to extend her life. There are a number of bloodlines in the nation, but the "pure" stock (the pale-skinned ones) came from another plane 2,000 years before. They are distantly related to the Flaems in Glantri, who also came from the plane of Old Alphatia. See, the Alphatians were the Followers of Air and the Flaems were the Followers of Fire, and they engaged in a debate about which was better until Alphaks (the then-ruler) declared arbitrarily fire had one. This started a war that destroyed the plane and banished the Flaems to wander the planes for centuries, while the Alphatians relocated to the Known World. Naturally, there is some hostility between Glantri and Alphatia (more of a rivalry), and definitely some between Thyatis and Alphatia because they're both massive empires right next to each other.
I could say more, but I'll stop here for now.
