Dungeons & Dragons retro-clones are restatements of Dungeons & Dragons rule editions no longer supported by Wizards of the Coast. They are mostly made possible by the terms of the Open Game License and System Reference Document, which allows the use of much of the proprietary terminology of Dungeons & Dragons that might otherwise collectively constitute a copyright infringement. While these rules lack the name Dungeons & Dragons or any of the associated trademarks, their intent is to have a playable experience similar to those older editions.
Retro-clones also are widely referred to as "OSR", "Old School", or "Old School Renaissance" games. In general, retro-clones are considered an established, accepted part of the hobby and they hold a particular fascination for many gamers.
Retro-clones can be (more or less) distributed into three broad categories:
There obviously is much overlap and interplay between these broad categories, especially when it comes to "re-imagining" them into a new system. Many retro-clone games draw freely from multiple sources. Most retro-clone games seek to preserve the "look and feel" of the originals while offering something additional or new - however, some implementations seek to preserve the originals as closely as possible with a minimal amount of innovation. Both approaches are valid and interesting.
A further complication arises when well-known and well-established retro-clones are themselves used as the basis of another retro-clone, resulting in a "two steps removed" type of game experience. For example, there are many re-implementations of the notable retro-clone Swords & Wizardry - see Swords & Wizardry and Compatible Products. To a somewhat lesser extent, the notable retro-clone Labyrinth Lord has itself served as the basis for several other games - see Labyrinth Lord Compatible.