Player Character Classes (DMG)

The Player's Handbook covers the nuts and bolts of character classes, explaining the mechanics of how they work and what they can do, but there is more to being a DM than just knowing the hard and fast rules. Character classes form the heart of the AD&D game, so it is useful to understand some of the concepts and relationships that define classes and how they function.
==Class, Level, and the Common Man==
Character class and level are useful game measures of a character's talents and abilities. Every class outlines a basic role for the character, a position and career in life. Each level defines additional power and provides a system whereby you can quantify and balance encounters.
With only a little practice you learn that characters of X classes and levels can easily defeat monster Y, but that monster Z will give them serious problems. This helps you create exciting, balanced adventures for your players.
Yet, at the same time, you know that the concept of classes and levels doesn't really apply to the real world. The teamster driving the wagon that passes the characters isn't a 1st-, 5th-, or 100th-level teamster. He is a man, whose job it is to drive wagons and haul goods. The chambermaid is not a special class, nor are her abilities defined by levels.
The teamster or chambermaid may be exceptionally skilled and competent, but for them this is not measured in character classes. There is no such thing as a teamster or chambermaid class, any more than there are merchant, sailor, prince, blacksmith, hermit, navigator, tinker, beggar, gypsy, or clerk classes. These are the things people do, not all-encompassing descriptions.
Nor are all the people in your campaign world fighters, mages, thieves, or whatever. The situation would be utterly ridiculous if every NPC had a character class. You would have fighter chambermaids, mage teamsters, thief merchants, and ranger children. The whole thing defies logic and boggles the mind. Most non-player characters are people, just people, and nothing more.
Only a few people actually attain any character level. Not every soldier who fights in a war becomes a fighter. Not every urchin who steals an apple from the marketplace becomes a thief. The characters with classes and levels have them because they are in some way special.
This specialness has nothing to do with ability scores, class abilities, or levels. Such characters are special by definition. The fact that player characters are controlled by players renders them special. Perhaps these special characters are more driven or have some unknown inner spark or just the right combination of talents and desires. That's up to the players. Similarly, non-player characters with classes are special because the DM says so. Plain and simple. There is no secret reason for this—it just is.