A Retro-Clone System is any game, currently in production and/or available legally for free, which tries to reproduce or support the look, feel, game mechanics and/or style of the classic Basic Dungeons & Dragons or Advanced Dungeons & Dragons games and the furtherance of the Retro-Gaming Movement.
Basic Fantasy, OSRIC, Swords & Wizardry, and Labyrinth Lord all qualify as Retro-Clone Systems - so each have their own section here.
Even though HackMaster and Castles & Crusades also fulfil the definition, they have their own forums here at the Realm - and all posts and discussions should be placed there.