IC: Dungeon Callers
After the White Box set, they wrote up a simplified version as a sort of introductory set (also the material and box art allowed it to be sold in toy stores), calling it "Basic D&D". It introduced dungeon crawling, simple characters, and only went up to 3rd level. Then they added the Expert rules which upped the level cap to 14th and introduced wilderness exploration.
The next addition was the Companion rules, which increased the level cap to 25. It introduced the idea of high-level characters becoming sovereigns, with lands of their own, strongholds, armies, politics, all that. The characters aren't required to be together all the time any more, as they're each strong enough to act alone if they choose. (In other words, they might each start on their own direction in the campaign.)
The Master set increases the level cap to 36 (the maximum), and introduces planar travel and attempting to gain true immortality -- as in, attaining deity status. And that is a ruleset in itself, the Immortals rules.
All together, that set is referred to as "B/X" or "BECMI". A compilation of these rules was put into the Rules Cyclopedia back in '92 or so, right about the same time that 2nd Edition D&D came out. It evolved into a separate game, with its own rules and quirks.
This is the version I've been calling "Classic D&D" for simplicity. Calling it BECMI might make some people think it's not D&D.