Appearing in a primitive form in the second edition rules of the Dungeons and Dragons RPG and developed extensively in the third edition rules, Prestige Classes (PrC) are character classes that offer specialized, exclusive abilities once certain restrictive requirements are met.
In its original form and in the first edition "Advanced Dungeons and Dragons" rules -- in which the term "prestige class" was not used -- subclasses often required a character to have higher ability scores or specific alignments as opposed to those allowed in the standard classes. For example, a character seeking success as a Thief required a relatively high Dexterity score, but a character seeking to enter the Assassin subclass needed an exceptional Dexterity score as well as adherance to an evil-attuned alignment. A similar relationship existed between the Cleric and the Druid, and the Magic-User and the Illusionist. Meeting the more stringent subclass requirements granted access to advanced skills and different (often more specialized) spells and abilities than were available via the parent class. A handful of other classes, while nominally subclasses, so blended the abilities of two or more classes or subclasses that they essentially stood alone. These classes -- including the Ranger (combining elements of the Druid, Fighter, and Magic-User), Paladin (combining elements of the Fighter and the Cleric and adding substantially different abilities not seen in any other class), Monk (combining elements of the Cleric and the Thief plus new and different abilities) and Bard (combining elements of the Fighter, Thief and Magic-User plus other new abilities), and later the Cavalier and Barbarian -- had complex requirements, including exceptional ability scores, narrowed alignment choices, armor and weapon restrictions, and often dictated how the character's funds must be distributed. These classes were the forerunners of the prestige class.
The concept was developed further in the second edition rules. At that time, even more highly specialized subclasses emerged. In addition, the dual-class and multi-class rules (via which a character could change classes while retaining some of the abilities of the former class, or could pursue multiple classes simultaneously) were streamlined and refined from the rather awkward and limiting form they took in the first edition rules. The stage was set for the unification of extended multi-classing possibilities with elite stand-alone classes that offered exclusive special abilities and skills.
The result that emerged from the third edition rules was the Prestige Class. In addition to the kinds of restrictions normally required by subclasses, PrCs often require that a character already have a certain number of experience levels in one or more classes -- a new character cannot simply begin life as a member of a prestige class. Often a specified minimum number of ranks in particular skills, abilities or feats are also required. Gaining experience in a PrC is similar to taking on a new or second class. The role-play effect in the game is not so much that of changing careers (as with dual classing) or combining multiple classes, but is more like achieving recognition in the character's original class or classes and being allowed to join an exclusive brotherhood of similarly skilled individuals.
Most PrCs are focused on specific goals (e.g., seeking out and destroying evil creatures, defending the community, undermining authority, achieving ultimate mastery over specific weapons, etc.) and require that the character act accordingly (most standard subclasses don't have such expectations). In addition, the PrC membership allows for a broad range of character nuance, depending on the classes/skills/abilities/feats that the character brings with it into the PrC. A character party consisting solely of members of the same PrC (assuming the PrC has no restrictions against such a gathering) would likely guarantee a community of common purpose and like-mindedness while still offering an interesting variety of skills and abilities.
Among the PrCs available in the third edition rules are Arcane Archer, Assassin, Blackguard, Shadowdancer, Harper Scout, Champion of Torm, Dwarven Defender, Pale Master, Red Dragon Disciple, Shifter, and Weapon Master.