Short answers, pulled from the story.
Post-classical history is the period from about 500 CE to about 1450 or 1500 CE, roughly corresponding to the European Middle Ages after the decline of the western Roman Empire. World historians use it as one of five or six major periods, sitting between classical societies and the early modern world.
Post-classical history is also called the medieval era, the post-antiquity era, the post-ancient era, and the pre-modern era. Historians adopted the term post-classical in part to avoid the loaded assumptions tied to terms like Middle Ages, Medieval Period, and the Dark Ages, though applying it globally has been criticized as Eurocentric.
The world population broadly doubled during post-classical history, rising from approximately 210 million in 500 CE to some 461 million in 1500 CE. Growth was slow and repeatedly set back by events like the Plague of Justinian, the Mongol invasions, and the Black Death.
The three major universalizing missionary religions of the post-classical period were Buddhism, Christianity, and Islam. Buddhism spread from India into China and on to Japan, Korea, and Vietnam, Christianity spread through Europe, and Islam spread from Arabia across North Africa, Persia, Central Asia, India, and Indonesia.
The Mongol Empire, active in the 13th and 14th centuries, was the largest continuous land empire in history and enforced a stability that revived Silk Road trade across Eurasia. Founded under Genghis Khan, proclaimed ruler of all Mongols in 1206, it fractured into four khanates by 1294 and had collapsed by 1500.
Two great plague pandemics struck during post-classical history. The Plague of Justinian of 541 to 549 killed a quarter of the Mediterranean's population in its 542 European outbreak, and the Black Death of 1347 to 1351 killed between 25 and 50 percent of populations.