Warriors

Fighters, what is there to say about their place in a default D&D world, they hit things with weapons.  Warriors and weapons are more common in a fantasy world than in medieval history due to the constant threat of goblins, beasts, and other threats.   Any peasant can quickly find themselves skills in arms and armor without formal martial training.

In our default world, Fighters can be an individual commoner who has had to take up arms against threats, part of an informal town guard, or a soldier with formal training.  The trick is the other warrior classes (Rangers, Barbarians, Paladins, and various sub classes.

Barbarians: The barbarian class is a misnomer, particularly in modern D&D.  The name of the class denotes more a a cultural trapping, but the mechanics of the class reflect a reckless "raging" fighting style.  Members of this class can hale from a distance, rugged land, but they can also be a local with anger issues.

Rangers:  I have discussed some of my perceived issues with the Rangers before.The abilities of the class do denote some additional level of special training or exposure pertaining to the wilderness and the threats therein. There are troupes of Rangers in the more traditional sense, groups of individuals who specifically work to keep the threats of the wildlands from infringing on the bounds of civilization. 

Paladins: Much of what can be said about the Paladin was covered in my discussion of Clerics. They may be thought of as Clerics who know weapons a little better, they may be individual crusaders, or members of some formal organization.  In any even The Church will likely claim them.

Fighters: while lacking many of the class abilities of the above, can also easily be found among their ranks and worthy of the title (even if not members of the class). 

Magic and Warriors: Many classes, particularly in modern D&D have access to spells and magic abilities.  I don't have a specific philosophy when it comes to these matters.  They can be the result of learned study, passed down traditions, self discovery, or some other diegetic source.  It works best for the nature of the modern D&D game to allow players to advance without being anchored to a specific facet of the world.