These are characters which affect the player's story in some way which normal players are not running in the game.

These are major non player characters that are important to the campaign being run, their actions influence the campaign. In fact, if they were a different person, the campaign would be different.
Character Sheet        
This category is for PCs involved in forum RP
These are members of the background that fill the world behind the player characters. They are needed to give verisimilitude and life to the game world the players are passing through. Any Extra character would be either a typical character or a nearly typical (just different to stand out slightly) character. The players might have a line or two of dialog with them. If the extra is interesting, there might be more interaction with them, and the extra becomes the springboard for minor subplots in the game.
These are groups where a single character sheet and description defines (nearly) every member of the group. This would be for dockside bandits, Ninjas, pitchfork and torch wielding peasants, BlackMoor Barbarians Riders, or some other loose group of near identical people. Hordes are almost always antagonistic.
These are characters that are of major importance in this chapter of the characters' existence. They are either villains (master minds, Leaders, major minion), major allies (supporting characters), or Color characters (The Archmage, The Leader of the Living Trees, ). Most of the "name characters" in the Lord of the Rings are Major ones.
These are characters that are of minor importance in this chapter of the characters' existence. They are either minor villains (important minions, lieutenants, monsters of some note), minor allies (supporting characters), or Color characters (the local headman, a healer, soothsayer, the troll beneath the bridge) The line between Minor Color Characters and Extras is a little fuzzy. It all depends upon the character's presentation to the players.
Mythic/ Historical        
These are characters the players will never meet, but their existence has impacted the world. Even if they were not real (or were not quite what the legends pain them), they have made an impact upon their world. Think King Arthur, Merlin, Alexander the Great, Cesar, Noah, and more.
Scenario Based        
These are characters are used to drive a single scenario (or a few tightly linked ones). Once those scenarios are resolved, the character is no longer really important to the campaign.
This is a character who concentrates upon growing food and organic resources. The primary agricultural is the farmer, but it includes ranchers, field hands, moisture farmers, and harvesters. This is not a very heroic orientation, but sometimes your NPC is a farmer.
These is a character who concentrates on the creative area of living, both performance arts, writing, and producing artistic things. These are groups of dancers, musicians, writers, bards, actors, and sculptors. It also includes collectors, critics, and patrons of the arts.
This is a character who concentrates upon warfare and personal combat. Soldiers, Mercenaries, Sargents, Tacticians, Generals, Military supply/ logistics, and so on.
This is a character who focuses upon stealth, infiltration, and/ or the taking of goods and information. This is for criminals, spies, assassins, and those who combat them, detectives, intelligence officers, and police.
Domestic/ Craft        
This is a character who focuses upon performing domestic task or other trade crafts. This includes servants, vintners, and seneschals.
This is a character who focuses upon wisdom and learning. They are a curious and investigative type. They gather knowledge for themselves and sometimes teach others.
This is a character who focuses upon trade, commerce, business of all types, and money. Salesmen, merchants, traders, brokers, and investors are of a mercantile focus.
This is a character who focuses upon thaumaturgy (magics of all types), powers of the mind, and the cosmic forces of the universe, including deities. Many of a mystical orientation are also known for theurgical knowledge - wisdom and understanding of the universe.
This is a character who focuses upon the skills useful for life in the wilds. This includes, and is not limited to, ability to move through the environment, ability to find food, ability to find water, ability to find shelter, and an understanding of flora and fauna.
This is a character who focuses upon political power, the process of government, and the leading of people. Suitable for politicians, ambassadors, courtiers, councilors, first speakers, leaders, kings, princes, dukes, and others who control the course of cities, states, and nations.
This is a character who focuses upon the theurgical, knowledge and wisdom of the universe and deities. Most of the religious orientation work with their community, doing the divine will. This is suitable for Shamans, WitchDoctors, Priests, Monks, and Clerics of all persuasions. Many of a religious orientation are also known for thaumaturgical knowledge and power (wonder working, magic, miracles).
This is a character who lives for the functional, mechanical, and technical. They are craftsmen, the makers of things, stone smiths, hackers, radio operators, warp engineers, trappers, motorheads, and others who live for machines and technology.
This is a character who lives for travelling by some means. Sailors, Pilots, Navigators, Submariners, Dragon Riders, Pony Express Riders, Drivers, and Charioteers, are all examples of Travelers.