NBT-data/NPC Inventory

While all of the normal NBT-data applies to all mobs, NPCs with inventories and gear have even more NBT tags that can apply. Mevans showed how each specific type of item is assigned in this post.

Equipment
Also found on this page, there are 5, non-named slots for equipment, used like such:  {Equipment: [{id: ____}, {id: ____}, {id: ____}, {id: ____}, {id: ____}}]  {} 0: The item being held in the mob's hand. {} 1: Armor (Feet) {} 2: Armor (Legs) {} 3: Armor (Chest) {} 4: Armor (Head)

Held Items
In the {NPCItemsInv:[]}, there are 12 different fields that can be specified, these being  {NpcItemsInv:[{Slot:0, ____}, {Slot:1, ____}, {Slot:2, ____}, {Slot:3, ____}, {Slot:4, ____}, {Slot:5, ____}, {Slot:6, ____}, {Slot:7, ____}, {Slot:8, ____}, {Slot:9, ____}, {Slot:10, ____}, {Slot:11, ____}]}  Each of these assigns a different item to a different "slot" of an NPC
 * Slot:0 - an NPC's Idle Item, what they hold while not in combat
 * Slot:1 - an NPC's Melee Weapon, what they use for melee combat
 * Slot:2 - an NPC's Ranged Weapon, what they use for ranged combat
 * Slot:3 - an NPC's Spear Backup, what they use after their spear has been thrown
 * Slot:4 - an NPC's Eating Backup, what they hold while they are eating
 * Slot:5 - an NPC's Idle Mounted Backup, what they hold while mounted
 * Slot:6 - an NPC's Melee Mounted Weapon, what they attack with while mounted
 * Slot:7 - an NPC's Replaced Idle Item, what they "remember" if you give an NPC custom gear
 * Slot:8 - an NPC's Replaced Mounted Melee Weapon, what they "remember" if you give custom gear to a mounted NPC
 * Slot:9 - an NPC's Replaced Mounted Ranged Weapon, what they "remember" if you give custom gear to a mounted archer
 * Slot:10 - an NPC's Bombing Idle Item, what bombardiers hold in their off-hand
 * Slot:11 - an NPC's Bomb, this specifies how strong the Orc Bomb is

Note that if you summon a Gondorian Baker with a bow in Slot 2, this will not automatically make him an archer. The slots are dependent on the type of NPC, not vice versa.

Replaced Items
With the new NPC format that allows players to replace NPCs' weapons and armour, there are 7 other NBT tags that can be applied to {HiredReplacedItems:[]} :  {HiredReplacedItems:[{Slot:0, ____}, {Slot:1, ____}, {Slot:2, ____}, {Slot:3, ____}, {Slot:4, ____}, {Slot:5, ____}, {Slot:6, ____}]}  These specify what an NPC has "underneath" player-given armour. These are not very useful, as the player has already given armour, but could possilby be used for role-play scenarios where a unit is "disguised" with another faction's armour.
 * {Slot:0} is the Helmet
 * {Slot:1} is the Chestplate
 * {Slot:2} is the Leggings
 * {Slot:3} is the Boots
 * {Slot:4} is the Melee Weapon
 * {Slot:5} is the Orc Bomb
 * {Slot:6} has no use yet, but will be for a Ranged Weapon

Examples
This may all seem very confusing, but with a little practice, you could be creating your own well-equipped NPC. Here are a few explained examples to help break down parts of the NBT tag. lotr_summon lotr.RohirrimArcher ~ ~ ~ {Equipment: [{}, {id:314}, {id:315}, {id:316}, {id:317}]} This will summon a Rohirrim Archer, with nothing in his hand, wearing Golden Armour. Note that since Item IDs are not fixed, using the name is also acceptable, such as lotr_summon lotr.RohirrimArcher ~ ~ ~ {Equipment: [{}, {minecraft:golden_helmet}, {minecraft:golden_chestplate}, {minecraft:golden_leggings}, {minecraft:golden_boots}]} This makes it easier to understand, and can be copied from world to world.