1975: Supplement I: Greyhawk is published, which makes some major revisions to the rules. (The original rules pretty much only use d20s and d6s). It adds in different hit dice and weapon dice, and a lot of other things. A major change is the way it handles ability scores – the original had hardly any bonuses due to them (no bonuses from Strength, for instance), but this expands the range of bonuses significantly. Further supplements (Blackmoor, Eldritch Wizardry) expand the game, but they don’t change it as much as Greyhawk does.
1977: Basic Dungeons & Dragons, edited by J. Eric Holmes. It’s an introduction to original D&D with a few rules from Greyhawk included. The ability score modifiers follow on original D&D without the Greyhawk adjustments. It covers levels 1-3.
1977-9: Advanced Dungeons & Dragons. This is a revision of the original D&D with all the supplements included. It uses the Greyhawk ability score modifiers with some adjustments, and has classes that were added in the supplements or in the magazines (The Strategic Review and The Dragon).
1981: Basic Dungeons & Dragons, edited by Tom Moldvay. This is a major clean-up of the rules. It is based on the original D&D games but makes a number of changes (for instance, ability score modifiers are +1 for 13-15, +2 for 16-17, and +3 for 18). It is complemented by a D&D Expert set by David Cook (levels 4-14). This is the version of D&D I learnt off, and it is generally regarded as one of the best expressions of the rules.
1983: Basic Dungeons & Dragons, edited by Frank Mentzer. This one is the famous “Red Box”. It’s basically just the Tom Moldvay rules formatted slightly differently with Player and Dungeon Master books. Although this is the best-selling version of the rules, I don’t like it as much as the 1981 version. There are a few rules changes in the Expert book so that higher level play in the Companion (15-25) and Master (26-36) levels could be supported. Eventually an Immortals sets of rules came out to handle post-36th level play.
1989: Advanced Dungeons & Dragons 2nd Edition. A major revamp of the main D&D game. Although the mathematical foundation is mostly the same, a lot of how the classes worked changed, and the rules were presented in a much clearer form. Gygax was good at inspirational writing, but rarely at writing clear rules.
1991: Basic Dungeons & Dragons Black Box. It’s really just the 1983 box rules again in another box, but covering 1st to 5th level. Why was it needed? Because of the next product…
1991: Dungeons & Dragons Rules Cyclopedia. This is the big one. Compiling the Mentzer Basic, Expert, Companion and Master sets into one book. It’s comprehensive. It’s not really for the beginner (that’s what the Black Box was for). The Immortal rules were completely rewritten in Wrath of the Immortals, but by this time the Basic line was on the way out. A couple more products and, by 1993, the line ended, with only a couple more products being released.
1995: AD&D Player’s Options. Although not actually a new edition as you still needed the 2E books, these three books (Combat & Tactics, Skills & Power and Spells & Magic) gave a lot of optional ways of running the game. They used (basically) the same mathematical underpinning as 2E, but added a LOT of options for players and DM in how to handle combat, build new classes and use magic. Occasionally referred to as 2.5E, they are also of dubious balance – though I loved them (especially Combat & Tactics) at the time.
2000: Dungeons & Dragons 3rd Edition. And we get to the modern (Wizards) era of the game. A huge update of the AD&D rules, with the “advanced” being lost because it no longer made sense. (It’s intimidating for new players, for one thing). It’s definitely a member of the AD&D line, however. Tremendously popular, and powerful. And more than a little complicated – although that’s mainly due to the number of options, because the basic rules are fairly clear.
2003: Dungeons & Dragons 3.5 Edition. Lots of small changes make a .5 revision!
2008: Dungeons & Dragons 4th Edition. Another massive change in the rules. It wasn’t that successful, but pioneered a lot of interesting ways of handling the game.
2010: Dungeons & Dragons Essentials. Not exactly a new edition, but another way of approaching 4E. This take on the classes is pretty good, but it couldn’t help the terminal state the game was in.
2014: Dungeons & Dragons 5th Edition. Where we are now! Another massive change to the rules, but drawing much more on older editions. And recent editions. It takes the best of the various editions of the game and is really fun to play!