Short answers, pulled from the story.
A character class defines a player character's capabilities, strengths, and weaknesses in Dungeons & Dragons. Choosing a class is one of the first steps in creating a character and determines available skills, abilities, and combat roles throughout the game.
The original Dungeons & Dragons boxed set included three classes: the Cleric, the Fighting-Man, and the Magic-User. The first supplement, Greyhawk, added the Thief as a fourth class and the Paladin as a Fighting-Man subclass.
Prestige classes were introduced in the 3rd edition Dungeon Master's Guide. They are inaccessible at first level and require characters to meet specific prerequisites, such as certain feats or membership in a specific organization, before entering them.
Fighters were the most created class in 5th edition, based on D&D Beyond data from August 15 to the 15th of September 2017, with 13,906 combinations per 100,000 characters. Rogues ranked second at 11,307 and wizards third at 9,855. Druids were the least created at 6,328.
The 4th edition defined four combat roles for character classes: Leaders, who focused on buffing and healing allies; Controllers, who affected multiple targets or altered the battlefield; Defenders, who absorbed enemy attacks; and Strikers, who dealt heavy damage to single targets while staying mobile.
Multiclassing rules varied significantly across editions. In 1st and 2nd editions, only humans could change classes and it required extremely high ability scores. The 3rd edition introduced a unified experience table allowing players to freely mix class levels, while the 4th edition replaced prestige classes with paragon paths and epic destinies. The 5th edition requires minimum ability scores of 13 in relevant stats to multiclass.