Once upon a time, there were two Dungeons and Dragons; it was in a time before Pathfinder and D&D 4e were competing for supremacy. One was called Advanced Dungeons & Dragons, the other was simply Dungeons & Dragons. Sometimes it was called 'Basic' D&D because of the existence of 'Advanced', other times it was called BECMI D&D. Or it was named after the people who edited the rules for a given edition.
It might surprise younger readers that Dungeons & Dragons had five editions between 1974 and 1994. These were evolutionary changes rather than revolutionary, small fixes and clarifications to the game rules instead of vastly different versions of the rules. Want to know more? See you after the jump.
This 'basic' version of D&D has its roots in the original 1974 rules. A enthusiastic hobbyist named John Eric Holmes approached TSR about reworking those older rules into an 'introductory' version of D&D. TSR saw this as a great way to hype the release of AD&D and make some money while they were at it. This began the tradition of a 'Basic Set' that includes introductory rules, dice, and an adventure module, incidentally. The Basic Set was released in 1977, being a 'second edition' of the original game that came in little white booklets. It also helped players deal with the fact that AD&D wasn't going to be fully released until 1979 with the publication of the Dungeon Master's Guide. It also turns out it wasn't much of an introductory product, even though it did attempt to explain the how of playing D&D (which is something I noted that 1st Edition AD&D didn't attempt much of). Instead it was a transitional product, meant to bridge the gap between the original D&D game and the forthcoming AD&D game. Even that didn't work out very well because the AD&D rules turned out to be very different, and somehow players were expected to just start playing AD&D after reaching 4th level
The ploy worked very well and the Basic Set brought in new players to the hobby. It might have worked slightly too well, since the 'transition' product was alive and well once AD&D 1E had finished releasing. TSR felt it would be a good idea to continue to have an introductory rule set, since it's a fundamental rule of business that expanding your customer base is a good thing. In 1981, Tom Moldvay revised the rules and added an Expert Set to cover levels 4-14, important to know since the Basic Set only covered levels 1-3. But a strange thing happened once this had been finished; the 'basic' game became its own game. While TSR still saw it as something to guide people into AD&D, that's not what was really happening. Moldvay's D&D was a different beast than its bigger brother, and people liked it. So much so that the next head developer of the line, Frank Mentzer, expanded everything into five boxed sets: Basic Rules (levels 1-3), Expert Rules (levels 4-14), Companion Rules (levels 15-25), Master Rules (levels 26-36), and Immortal Rules (godhood, basically).
During this period, adventure modules were published to support the game, and a shared campaign setting emerged from those adventure modules. Although it initially had no name other than the Known World (or the Grand Duchy of Karameikos), eventually it was given a name: Mystara. A series of gazetteers were published to flesh out the campaign setting. Eventually TSR got around to putting out a boxed set style campaign setting, much like they had been doing with AD&D. It was actually an expansion of Mystara! It turns out that Mystara is hollow, and the Immortals (the deities of the setting) had hidden away a bunch of plants, animals, and cultures that were in danger of becoming extinct on the outer surface of Mystara. The setting was called the Hollow World, and was supported by a few adventures and setting books.
Eventually the rules were all collected and revised into one volume by Aaron Allston, called the D&D Rules Cyclopedia, in 1991. It was supported by a boxed set as well; a huge black box with a extremely large red dragon menacing a fighter on the front. That's the set I remember getting back when my dad decided to buy the thing from the local KB Toys. The last hurrah for Classic/Basic D&D was in 1994 with the publication of the Classic Dungeons & Dragons boxed set.
Funny story though. This line of D&D had a major influence on what would become D&D 3rd Edition, because as far as I know, it was the first version of D&D to normalize the ability score modifiers. That is, no matter what the ability score was, you knew that a 15 was a +1 to whatever things that ability score affects. In AD&D, things were not so consistent, and a 15 in one score might not mean anything or be a big deal in a different one. We can see Basic D&D's influence in 5e as well, in that it offers streamlined play while being modular enough to accommodate those who want more complexity.
How strange that a 'basic' D&D game possesses influence on the D&D brand even to this day.