Advanced Dungeons and Dragons the RPG, published by TSR was for a very long time the Windows of the RPG world, the default standard system that just about everyone (love it or hate it) used

Advanced Dungeons & Dragons 1st edition (AD&D 1st Ed) was origionaly published as simply a collection of the many archaic rules which had by then accumulated for the D&D system. It was released in two hardback books; the slim Player's Handbook (PHB) detailing all you needed to make a character and play the game, and the vast Dungeon Master's Guide (DMG) detailing useful information for creating worlds and adventures. Information included in the DMG included geology, theology, psychology, cartography, astrology, astronomy, metaphysics and what to do if you find yourself on post-holocaust Earth.

2nd Ed AD&D (also called Blue Edition due to the colour of the interior artwork) vastly expanded the PHB while removing some of the more unusual (some might say interesting) classes and options. Gone were the (Eastern Fighting) Monk, the Assasin and the Psionicist. There was also no sign of the popular classes such as the Barbarian that had been added in Unearthed Arcania (1st Ed's answer to Skills & Powers). This was due to 2nd Ed's focus on more 'standard' fantasy settings. Some aspects (eg. psionics) were added in in later expansions, of which there were many. 2nd Ed clarified many aspects of the complex combat system, resulting in the fact that you no longer had to leaf through look up tables everytime you entered combat; though for new players the stat called THAC0 (the 'to hit' stat) may have looked daunting.

2nd Ed's failing was that its core rules were not updated. This allowed competiors to learn from AD&D, find it's faults and then produce streamlined copies, which were in turn built upon until they evolved into an entirely new generation of RPGs, while AD&D merely released rules suplements which managed to add to the confusion ("I know it says that in the PHB, but if you look it up on table X15 in Munchkins of the Underdark and adjust the modifier with Rary's Obscure Reference from GH555 you'll see I do 1d8+7921hp damage with my toe"). The only reasion AD&D 2nd ed managed to last as long as it did was the support for it's campaign settings. With great fantasy worlds such as Dragonlance, Greyhawk and Forgotten Realms as well as such imaginative creations as Spelljammer, Ravenloft, Dark Sun and Planescape the AD&D line had huge drawing power for almost every aspect of the fantasy genre. Despite this sales of the rule books dropped slowly but surely. An attempt was made to boost sales by overhauling the look of the 2nd ed PHB and DMG, but the guys at TSR must have been living in one of their own fantasy worlds along with their giant space hamsters, at a time when just about everyone agreed the rules needed reforming they plastered a re-release of the core rule books with assurances telling us not to worry as they would never dream of bringing out a 3rd Ed.

AD&D remained the 'default' language for gamers for a very long time; if you wanted to tell someone about your Shadowrun PC but your mate knew nothing about the system, you knew he'd understand AD&D stats ("he had, like, strength 18 man!"=Shadowrun Str6). Eventualy players jumped ship, mainly to the more streamlined Storyteller system, a fact that can be observed by conducting the following test: Get a random gamer; ask them if they would like their next character to have Strength 5. Most modern gamers will assume Str5 is good, which it is in Storyteller. Of course if your gamer is a real newbie or an old timer then he might well envision the AD&D definition of someone with hardly the strength to walk.

AD&D (the RPG) has now been officialy discontinued as a brand as the new owners Wizards of the Coast of Magic: The Gathering and Pokemon fame finaly realised that with the demise of the origional D&D there wasn't anything for it to be advanced from. What any long term gamer would recognise as (and probably accidently reffer to as) AD&D 3rd Ed is now just called D&D and is part of the d20 system.