Short answers, pulled from the story.
Dungeons & Dragons retro-clones are fantasy role-playing games designed to emulate earlier editions of D&D that are no longer supported by Wizards of the Coast. They are made possible by the Open Game License, which allows designers to use much of the proprietary terminology from D&D without infringing copyright, though the games cannot use the trademarked names Dungeons & Dragons or D&D.
OSRIC, which stands for Old School Reference and Index Compilation, is widely regarded as one of the first and most successful D&D retro-clones. It was initially authored by Matthew Finch, later developed by Stuart Marshall, and released to the public in 2006 as a recreation of the first edition of Advanced Dungeons & Dragons.
Labyrinth Lord is a retro-clone written and edited by Daniel Proctor and published by Goblinoid Games in 2007. It emulates the 1981 Dungeons & Dragons Basic Set edited by Tom Moldvay and the accompanying Expert Set by David "Zeb" Cook, extending character advancement to 20th level compared to the original Expert Set's cap of 14th level.
Swords & Wizardry, developed by Mythmere Games, emulates the original 1974 edition of Dungeons & Dragons. It won the Silver for the 2009 ENnie Award for Best Free Product and is available in three versions: White Box, Core, and Complete.
Old-School Essentials, originally called B/X Essentials, is a retro-clone of the 1981 Dungeons & Dragons Basic and Expert sets. It is unique in offering both its Classic Fantasy and Advanced Fantasy versions in two formats: all-in-one rules tomes or modular multi-book sets. As of 2020 it comes in two versions, with the Advanced Fantasy version incorporating B/X-compatible material from first-edition Advanced Dungeons & Dragons.
The Old School Renaissance, or OSR, is a community movement built around retro-clones and older-edition D&D playstyles. Supported through online forums and blogs, it grew beyond digital distribution and began placing retro-clone games in physical game stores alongside mainstream tabletop products.