Online:NPCs

NPC (Non-Player Character) is a generic term for all humanoids (of various races and classes), other than the player, that are encountered while playing The Elder Scrolls Online.

All creatures are also technically considered NPCs, but only those that resemble humanoids are included in the NPC listings, usually differentiated by having a different race to other NPCs. The remaining creatures can be found on the creatures page. 

Named NPCs
There are many named NPCs in Elder Scrolls Online. All quest-givers and vendors in the game, as well most of the people in cities and towns are named characters.

Generic NPCs
There are also many generic NPCs in the game, most of which are randomly generated. If one dies for whatever reason, the game will eventually generate a new random replacement. There are friendly generic NPCs, including citizens, City Guard and workers, as well as hostile generic NPCs, including those who belong to various enemy factions. For the full list of generic NPCs, see Generic NPCs page.

Reaction
The term "Reaction" describes how an NPC interacts with you. An NPC's reaction to you also determines what color their health bar, nameplate and glowing NPC outline is if you have these UI options turned on.

Friendly: Friendly NPCs can't be attacked or pickpocketed. Some friendly NPCs (namely merchants) can report crimes, but they will not fight. Friendly NPCs that aren't merchants will not report crimes. If nameplates are turned on, their health bars, names and outlines are green.
Justice Neutral: Justice Neutral NPCs can be attacked and pickpocketed with consequence. They will report crimes if they witness them, and will fight back if attacked. They can also be provoked into attacking if they witness a crime being committed, but they won't always do so. If they witness a murder, they will report it, and some will attack you or flee the scene. They have yellow health bars and white nameplates and outlines.
Neutral: Neutral NPCs can be attacked without consequences and will not report crimes. They will fight back if provoked. Neutral NPCs cannot be pickpocketed. Their names, health bars and glowing outlines are yellow.
Hostile: Hostile NPCs can be attacked without consequence and will not report crimes. They attack on sight. Hostile NPCs have red health bars, names and outlines.
None: Usually reserved for deceased characters, also applies to NPCs without healthbars that can't be interacted with in any way (coded as "0")
Player Ally:   (coded as "4")
NPC Ally:   (coded as "5")
Companion: Used only for Companions (coded as "6")

Notes
Most quests will be given by NPCs, but you can still pick up many quests from books, notes, corpses, and locations.
Companions were introduced as permanent "follower" NPCs in Update 30. There are also quest followers, who will follow you for the duration of particular quests and usually only within a small radius of the quest location, and Assistants, who do not get involved in conflict.
At its launch on April 4th, 2014, the game earned the Guinness World record for the most unique NPCs in a videogame.  At the time, it was at a total count of 10,202, but has now risen to between 18,920 unique NPCs (22,126 if including generic NPCs, and 24,465 total if additionally including creatures).

Bugs
 
Logging off and logging on again, or simply waiting a few seconds for the NPCs to render, may fix this.
If you are on a quest, and the above fix has not worked, abandon the quest and log off for 15 minutes. See Resetting Quests for more details.
References