After the success of OD&D (Original Dungeons & Dragons), TSR (Tactical Studies Rules) released a more expanded version of Dungeons & Dragons, known as the Basic Set. At the same time, a different version, AD&D (Advanced Dungeons & Dragons) was released. These two versions exist because of the different creative visions Gary Gygax and Dave Arneson had for D&D. The Basic Set was coauthored by Gary Gygax and Dave Arneson, whereas AD&D was authored by Gary Gygax.
With more detailed rules and more structured gameplay, AD&D is divergent enough that many rules incompatibilities exist and is thus considered an entirely different game.