What is an NPC?
NPC stands for "Non-Player Character." Computer-controlled characters, such as questgivers, vendors, guards, bankers, and trainers are all considered NPCs. NPCs are either neutral, meaning both factions can interact with them, or they have a faction affiliation to the Horde or Alliance.

Can I see famous NPCs such as Thrall?
There are many well-known characters in the World of Warcraft universe that can be found around Azeroth and Outland. Along with Thrall, you can also find heroes such as Jaina Proudmoore, Sylvanas Windrunner, Cairne Bloodhoof, and many more. Pay them a visit and capture a screenshot for posterity!

Can you attack NPCs?
You cannot attack NPCs and shopkeepers in your own faction. However, you can attack NPCs and shopkeepers from the opposing side, as well as some neutral NPCs. Many neutral NPCs are affiliated with a particular reputation, such as the Argent Dawn NPCs. You must be "at war" with a group (found in the Reputations tab of your character sheet) in order to attack their NPCs.

For opposing faction NPCs, keep in mind that they typically reside in the opposite faction's towns and cities. On PvP realms, enemy players will be able to attack you if you enter their towns and cities. On normal realms, attacking an NPC or shopkeeper will flag you for PvP and enable enemy players to attack you. However, defeating a prominent NPC such as a racial leader (which include the likes of Thrall, Jaina, and Sylvanas) is a memorable accomplishment.

Do NPCs remember if you talked to them before?
For quest purposes, yes, NPCs do remember if you've talked. Thus, questgiver NPCs can keep track of your progress for quests they have given you.

Do NPC merchants close up shop at night?
No, however there are wandering merchants that might not always be where you last saw them.

Do shopkeepers have a limited supply of items?
Yes and no. Common items are always in supply. However, some shopkeepers sell uncommon items that may or may not be in stock when you visit their stores.