1. Description
An NPC (Non-Player Character) is any character you encounter and interact with in World of Warcraft that is not controlled by other human players — they are generated and controlled by the game server. NPCs are either neutral or affiliated with a particular faction ( Alliance or Horde). You cannot attack NPCs of your faction and you must be "at war" (set in Reputation tab of your character sheet) with a neutral NPC's faction to attack them. The monsters in the game are also NPCs, but they are mostly called mobs.

2. Trainers
A NPCs can be trainers, which means that they can give you skills in something. There are class trainers NPCs located in every of main cities for both factions. Stormwind, Ironforge, Darnassus and Exodar for the Alliance. And for the Horde Orgrimmar, Silvermoon, Thunderbluff and Undercity. From the class trainers you can learn new abilities which are unlocked with getting higher level and eventually by talents. Some abilities like the death knight's frost strike can be trained only if you has its talent. If you need to find a trainer or other npc in the city/village of friendly faction you just need to ask guards.
There are also profession trainers from them you learn a profession, which is used to create valuable items. You can have only 2 main professions at a time and as much as you want secondary professions. When you learn a profession its icon is in the general tab in your spell book from there you can create items. Your skill will be 1 first and when you create items it will increase. To make items you need to have materials for them. Some materials are sold by vendors, some are dropped by monsters and some can be aquired by one of your secondary professions. You can learn professions from many places in diffrent villages there are diffrent trainers you just need to type in google for which profession do you need trainer and it will show you the exact location.

3. Vendors
Vendors are NPCs that sell things from them you can buy mounts, armor, materials and etc... You can also sell items to them but ofcourse on low price. Some vendors can repair your broken items. These vendors are ussually weapon trainers, and armor sellers. You can use your tracking to find them easily.

4. Guardians
Certain items can temporarily summon a pet that will fight for you until it dies or its time limit expires. Some guardian pets are trinkets; these are usually reusable but limited by a cooldown time. A couple of guardian pets are offhand items, (e.g. Ancient Cornerstone Grimoire and Book of the Dead). One is a sword, Dragon's Call, which summons Emerald Dragon Whelps. Others are usable from your inventory but will either be consumable or have a limited number of uses. Guardian pets can be acquired in the following ways:
- Given as a quest reward for certain quest lines (for example, the Are We There, Yeti? quest sequence from Umi Rumplesnicker in Everlook rewards you with Mechanical Yeti).
- Reputation reward, e.g. the Defender of the Timbermaw trinket that summons a Timbermaw Ancestor requires Exalted reputation with Timbermaw Hold.
- Constructed by an Engineering schematic (e.g. the Gnomish Battle Chicken or Arcanite Dragonling) or Jewelcrafting recipe (e.g. Felsteel Boar or Truesilver Boar).
- Random area drops (e.g. the Glowing Cat Figurine that drops in Darkshore).
An early one that many people see is the Dog Whistle in Scarlet Monastery/Library, which drops off of Houndmaster Loksey, with 3 charges. Unlike Hunter and Warlock pets, there is no way to control a guardian pet other than by summoning it and/or waiting until it dies or its time expires. The pet will defend its owner, attacking whatever its owner attacks or whatever attacks the owner, which can lead to its attacking unintended targets before the party is ready for them. Caution is necessary before using such a pet in an instance. Use one only in confined spaces or areas where all but a few enemies have been killed, or just don't use it.

5. Mounts
Mounts are the basic way to player travel on level 20 you can buy a ground mount which will increase your speed with 60%. On 40 level you will have the ability to buy a 100% movment speed ground mount. On 60 you can get filght mount with 150% speed increase. And on 70 - 280% speed flying mount.
To buy these mounts you need to train a riding skill from riding trainers. When you are high enough level to buy the next mount you will recive an letter in your mail box which writes the location of the trainer and mount seller. You can summon flyght mounts only in Outland and Northrend and to summon in Northrend you need to buy Cold Weather Flying skill which costs 1000 gold. You can learn it on 77 level.