Golarion
Golarion is the "main" planet in Paizo's default campaign setting for the Pathfinder Roleplaying Game. It's not the sum total of the setting's Material Plane like, say, Toril is, but it's where most of the action is and where you're going to be spending most of your time so it's also the nickname for what's properly known as the Pathfinder Campaign Setting. Golarion was first introduced to players as a generic OGL module named Crown of the Kobold King, released in June of 2007 under Paizo's GameMastery imprint. Two months later at Gen Con 2007, Paizo published Rise of the Runelords, the first of Pathfinder's famous Adventure Paths and the first product to bear the Pathfinder brand name; this was followed by another adventure path and a couple "Pathfinder Chronicles" supplements. When Wizards announced Dungeons & Dragons 4th Edition and neglected to mention that the OGL would be replaced with a giant spiky dildo, Paizo cashed in on the confusion and announced that they would be creating a replacement RPG, based on OGL content but with a less restrictive license for its "Product Identity" and with Golarion as the default setting. Golarion is notable for ripping off fucking everything, including dime-store pulp novels, random bits of relatively modern history, earlier editions of D&D, sci-fi kitsch, and the kitchen sink. This is a transparent attempt to fill the gaping orifice left behind by the loss of the D&D's "Product Identity" as cheaply as possible, justified as a way of allowing many different campaign styles to coexist in the same setting. One notable exception is the elder gods of the Far Realm, which Paizo replaced by biting the bullet and licensing the Cthulhu Mythos from Chaosium.
The centerpiece of Golarion is the Inner Sea region, consisting of the fantasy equivalents of Western Europe, the Mediterranean Sea zone, and the northern half of Africa. Unlike most other fantasy settings, many of the cultures and civilizations of the Inner Sea region are in severe decline after the only deity which represents humans in the Great Beyond, Aroden, died a few centuries ago. To add salt to the wound, this caused a series of events which fucked up the world: the formation of a massive supernatural stationary fuckstorm that annihilated two entire nations and allowed pirates to develop their own kingdoms, the obliteration of a noble barbarian empire by a tear in the tissue of reality opened directly into the Abyss, and the prophets and diviners committing mass suicide as an prophesied golden age for mankind suddenly faded into nothing. As if this wasn't enough, the religious hysteria led the two greatest empires of the region to become balkanized devil-worshipping fantasy Nazis and decadent chucklefucks, respectively.
Nations of Golarion
Absalom
Mecca meets Sigil in the middle of the ocean. 10,000 years in Golarion's past, a fuckhuge magic space rock called the Starstone struck the planet, creating the Inner Sea in the process and destroying Azlant, home of the Glorious Azlanti Master Race. The last of the Azlanti, Aroden, lifted the stone out of the Inner Sea 5,000 years later and used its magic to become the God of Humanity in living flesh, founding the city-state of Absalom on the island where he dumped the thing to protect it from others who wanted to pull the same trick. It's still there, in the heart of the Ascendant Court, and anyone who survives the Test of the Starstone can use it to become a god too. (Thousands try every year; only three have ever succeeded, including one guy who took the test on a drunken dare and can't remember how he did it.) As everyone has a stake in keeping the number of crazy gods around to a minimum and the island is strategically located in the middle of the Inner Sea, Absalom has become the effective center of civilization in Golarion's western hemisphere, with a complete clusterfuck of races and nationalities walking the streets to wheel and deal. Other areas of note include the Cairnlands, the slightly-haunted graveyard for all the dumbasses who tried and failed to invade one of the most important places on the planet, and the Ivy District, which is where all the whores are and where everyone goes to stab each other in the back.
Alkenstar
Fuck yeah guns in your fantasy setting. Alkenstar is in the middle of a giant non-magical desert formed by a war between its magocratic neighbors, so they had to tell Medieval Stasis to shove it and learn engineering to stay relevant. Almost all of Golarion's gunpowder-based weaponry is produced by Alkenstari engineers, and they like to keep it that way so they can continue to charge out the ass for them. This is so your GM has an excuse to veto firearms. Despite what you'd expect from a desert nation that shits firearms, they're not particularly friends with Andoran, maintaining scrupulous neutrality and being situated in Golarion's equivalent of northeastern Africa.
Andoran
Fuck yeah 'Muricans.
Belkzen
Stupid dumb greenskin scum.
Brevoy
Asshole nobles struggling for dominance.
Cheliax
Motherfucking DEVIL NAZIS.
Druma
Jewish Scientologists. Contrary to expectations, not dwarven, though there is a substantial dwarven minority.
Galt
VIVE LA REVOLUTION!
Geb
Fuck yeah Lich King.
Hermea
Council of Wyrms: The Nation.
Isger
Cheliax's bottom bitch. Only notable because their flag has a barely-disguised swastika on it.
Gods of Golarion
Races of Golarion
Because Pathfinder is basically D&D 3.75, races should be considered their bog standard selves unless otherwise noted here.