This is some commentary to go with my illustrated guide to the editions of D&D.
All editions of D&D prior to the year 2000 were published by TSR, Inc. All of these editions are quite compatible.
I collectively call the various forms of D&D before the year 2000 that were not AD&D: classic D&D. This is after the practice of the Dragonsfoot Classic D&D forum.
O for original. The original game by E. Gary Gygax & Dave Arneson. I sometimes use a lowercase O since it wasn’t part of the original name. Some people call this “0e”.
Some people use “OD&D” for what I call classic D&D. (O for old?)
There was a need for an introductory version of D&D. Edited by J. Eric Holmes, the original D&D Basic Set filled that need. Intended as an introduction to either oD&D or AD&D, the Holmes rules are somewhat unique. The rules only cover 1st through 3rd levels.
The revised Basic Set edited by Tom Moldvay & the original Expert Set edited by Dave “Zeb” Cook with Steve Marsh. These formed a replacement for both the Holmes Basic Set & the oD&D box set.
The Basic Set still only covered 1st to 3rd levels. The Expert Set covered 4th to 14th & gave suggestions for handling higher levels. A Companion Set was promised to cover levels up to 36th.
(I believe that the concept of the Companion Set at this point was as a replacement for the oD&D Supplements. It would contain most of the oD&D material that hadn’t made it into the Basic or Expert Sets. This would change before a Companion Set was actually released.)
This Steve Marsh should not be confused with the other Steve Marsh, who works for Steve Jackson Games.
(This was the first roleplaying game I owned. It now is also my favorite edition of D&D.)
The Basic, Expert, Companion, Master, & Immortal Sets edited by Frank Mentzer. These replaced B/X & took PC to 36th level & beyond: to immortality!
The Rules Cyclopedia compiled by Aaron Allston combined nearly all the rules from BECMI as well as some additions from the D&D Gazetteers into a single volume. An introductory book covering 1st through 5th levels was also published to serve in the role of the old Basic Sets.
A for advanced.
AD&D was not a successor to D&D. The two lines were continued in parallel. AD&D was meant to be a more uniform form of the game & suitable for tournament play. D&D was kept as a freer, more malleable form of the game.
It is also rumored that the split occurred because of disagreements between Gygax & Arneson. While the D&D line continued to carry Arneson’s name, only Gygax’s appeared on AD&D.
O for original. (Sometimes I use a lowercase O to emphasize that it wasn’t part of the original name.) Also known retroactively as “first edition” or “1e”. Advanced Dungeons & Dragons was written by E. Gary Gygax.
Gary has said that there were many oAD&D rules that he seldom or never used: weapon vs. armor adjustments, psionics, &c. That he mostly included those things at the request of others.
After Gygax & TSR parted ways, Dave “Zeb” Cook was given the task of revising AD&D.
Gygax had planned a 2nd edition, though it would have been different from Cook’s. For one person’s take on what Gygax’s 2e might have been like, see Adventures Dark and Deep by Joseph Bloch.
After Wizards of the Coast bought TSR, they began work on a new edition of D&D. Since the new edition would draw inspiration from both the classic D&D & AD&D lines, & because the D&D line had been discontinued making the Advanced unnecessary; they called it just D&D.
Wizards of the Coast also released the basic rules behind 3e under a very liberal license (the OGL) as the d20 System. (Some people get the OGL confused with the d20 System License that involves the use of the d20 System logo.)
See also The Hypertext d20 SRD.
It has been observed that 3e is much more incompatible with previous D&D & AD&D editions than those previous editions are with each other. While some may find 3e a superior game, & despite the many traditions it held onto, there is no doubt that 3e is a very different game from AD&D or classic D&D. Some, therefore, are tempted to call it d20 Fantasy.
Created by Monte Cook, Skip Williams, & Jonathan Tweet.
Wizards of the Coast called this 3rd edition, though the books didn't actually carry that moniker. (They did carry the d20 System logo.) This refers to its place as a replacement for AD&D2e. Years before, however, TSR had referred to a later printing of the Holmes Basic Set as D&D 3rd edition!
Just three years after the launch of 3e, Wizards of the Coast released 3.5e. This time, the books actually bore the 3.5 moniker.
Of course, this emulation of software version numbers is flawed. In the software world, the dot in a version is just a separator, not a decimal point. Version 3.5 is not half way between versions 3.0 & 4.0. Version 3.5 should follow version 3.4 just as version 3.10 would follow version 3.9. That's OK, though, since foolish marketing departments for software companies have done exactly the same sort of idiocy.
By Rob Heinsoo, Andy Collins, and James Wyatt. 4e brought some significant changes. I hesitate to say much about it because I have not played it very much.
The latest game bearing the Dungeons & Dragons name seeks to appeal to fans of all the previous editions. A basic form of the game will be available as a free e-book (PDF).
Dan Proctor of Goblinoid Games wrote:
A retro-clone, the way I use the term, refers specifically to a game that attempts to emulate as closely as is legally possible the game rules of another game.
Why? There are several reasons. Here are a few.
There can be legal complications—real or perceived—when trying to publish material designed to work with someone else’s game. With the OGL (open game license), Wizards of the Coast made it clear how people could legally publish products to work with D&D 3e. A retro-clone using the OGL—or something similar—can likewise provide a clear way for people to publish products to support the cloned game.
Games go out-of-print and second-hand copies are finite and can be fragile. A clone can allow a game to live beyond the whims of an inactive owner, the needs of profitability, the aging of paper, or the occasional high prices in the collecting market. Even if there are plenty of affordable copies of a game available today, a clone can ensure that the game can outlive them.
Many of the clones also make it easy for an individual to copy and paste together a version of the game customized to their group and their campaign. Since most of the clones use the OGL or a similar license, it is easy to share such a document with your group online.
Beyond straight clones, there are also games that build on the ideas of earlier games but also explore new directions.
I haven’t tried to include every clone in my table but simply a few notable examples. And there are far too many clones and “more than clones” for me to catalog here.