Short answers, pulled from the story.
The original Dungeons & Dragons boxed set released in 1974 included the Cleric, the Fighting Man, and the Magic-User as its three character classes. These three archetypes formed the tactical backbone of the early game and defined how players approached conflict and survival.
The Greyhawk supplement released in 1975 added the Thief as a fourth main class and introduced the Paladin as a subclass of the Fighting Man. This expansion marked a significant shift in the game's design by incorporating more nuanced roles and abilities.
In the 1978 Advanced Dungeons & Dragons Player's Handbook, the Bard was a sixth class that required a character to start as a Fighter, change to a Thief, and then switch to become a Bard. This multi-step process reflected the game's increasing complexity and the desire of players to create more unique and specialized characters.
The second edition of Advanced Dungeons & Dragons released in 1989 grouped classes into four metaclasses: Warrior, Wizard, Priest, and Rogue. This simplification aimed to reduce confusion and contradictions while changing the Bard class to be a normal class that could be chosen at character creation.
The third edition of Dungeons & Dragons released in 2000 returned the Monk as a base class, created the new Sorcerer class, and included the Barbarian as a base Player's Handbook class. This edition also introduced prestige classes which were inaccessible at first level and meant to be multiclassed into from the base classes.
The fifth edition of Dungeons & Dragons released in 2014 introduced subclasses which allowed players to choose an archetype of their class at 3rd level or earlier. This edition also introduced new classes such as the Artificer which was added in the campaign setting book Eberron: Rising from the Last War.