Some NPCs are permanent, and always have the same model and appearance. Others are temporary, and their appearance may change, for example, after the zone instance was closed, or after a weekly/daily server reset.
A family is a group an NPC is associated with. This plays a role in Slayer achievements, and Slayer potions. On the wiki, race refers to the literal race or species of an NPC, such as a Crimson Moa belonging to the Moa race.
NPCs use profession skills and monster skills, with many monster skills that provide unique effects and animations. Monsters are not dependent on a weapon and do not have conventional skill bars like players. However, they must still use skills to attack, including normal attacks. Players can observe attacks that inflict damage through the "Combat" filter in the chat.
Most NPCs will have a level like player characters, although there are exceptions. All NPCs that can become hostile, including neutral and hostile NPCs, will have a level. The level of a foe is used to calculate the amount of experience a player gains upon its defeat.
NPC rank is an unofficial term describing the relative strength of NPCs of the same level. Stronger foes have names that are prefixed by Veteran, Elite, Champion, Legendary, or Epic to indicate they will be tougher to defeat than their normal counterparts. These foes also include a colored border around their portrait in the target indicator. NPC ranks are not always consistent with their actual strength, e.g. some Veteran NPCs might be relatively stronger than Elite.
Most NPCs have one or more abilities that describe skills available to the NPC. These abilities are descriptive rather than definitive, any monster that has a skill that inflicts bleeding may have a "Bleeding" ability and some monsters that inflict bleeding do not have an ability related to bleeding at all.
Friendly NPCs are known as allies. Skills that apply healing, boons, or other effects within a radius can affect friendly NPCs in addition to other players. In WvW, NPCs that are friendly to members of the same team are hostile to members of other teams.
Neutral NPCs are often called animals or ambient creatures due to these being the most common type of neutral NPC. Neutral NPCs are not hostile towards players or other NPCs unless they are attacked, and ambient creatures are usually defeated with one hit.
Hostile NPCs, known as foes, enemies, or mobs, will always attack the player characters or allied NPCs if they are aggroed. Hostile foes are identified by red text in their nameplate and target indicator. In the context of a boss encounter, if a high-ranking hostile NPC is aided by additional enemies, the latter are often informally called adds.