OSR stands for "Old School Revival" or "Old School Renaissance" depending on who you ask. In either case it refers to a movement that came out of dissatisfaction with third and fourth edition Dungeons and Dragons. When third edition D&D came out it was met with a mixed reaction from the community but it also introduced the Open Game License which allowed large portions of the rules to be borrowed and modified. Aside from opening the flood gates for third parties to publish content this meant that it was possible to publish ones own game using a lot of existing rules framework. Fourth edition arrived to near universal disappointment and bereft of their beloved classic D&D the older set struck out to recreate the game they loved.
While much of the OSR is the creation of retro-clones (systems that mirror "original fantasy role playing games" as closely as they can get away with) and related suplements it's also about recapturing the core feel of the earlier editions. Original D&D was a half finished mess of barely coherent rules that encouraged judges to fill in the gaps. The later editions of basic/expert and advanced D&D provided more but were still quite kludgy in places. What it lacked in elegance it made up for in creativity and strangeness. People playing D&D for the first time in the 70s and 80s remembered the thrill of discovering a new monster and the relentless mystery and wonder that suffused every new discovery. As time rolled forward and the D&D cannon was more or less set the dread of discovering a gelatinous cube or a lich slowly bled away to become cliche. Older players wanted their wonder and danger and through ten-thousand blog posts across hundreds of blogs a new ethos appeared.
Make it weird, make it wondrous, make it dangerous. The player should not know what they are facing, whether the rewards will be worth it, or whether their characters will live to the end. This is not the Lord of the Rings; it's Weird Tales! Your character will advance in power if they survive but you will not choose feats or optimize them. Attributes are rolled randomly, magic items are rolled randomly, the monsters may well be beyond your power. Fighting or fleeing is a real choice and if you play stupidly on the assumption that life is fair you are going to collide with an uncaring universe that doesn't think you are the main character. For all that the old school was tough it rewarded the clever and the lucky. The kingdom that you built on the corpses of a thousand orcs was earned and you knew it. You played an agent in a world not a character in a play.
Tonal assumptions aside the OSR is wide and varied. It comes from the gamer branch of a DIY culture that has everyone chatting and swapping ideas over the world wide web. Much of it is free and what isn't is usually reasonably priced. Dozens of systems like Labyrinth Lord, Dungeon Crawl Classics, and Swords and Wizardry exist and while they have their slight differences adventure modules typically port easily between them and exist in great quantity. For people coming back to Dungeons and Dragons later in life and discovering that the modern version is not to their liking the OSR offers much of the original flavor and content at a fraction of the price.
IRON NODER: WE'LL RUST WHEN WE'RE DEAD