The armory is currently separated into 11 different item types. Certain groups of item types can be safely grouped together (e.g. Guns and Bows both fit under the category of Ranged), though the specific categorizations are meant to help with builds (e.g. it is easier to swap a Melee in with another Melee than it is to swap with a Large Melee). The categories are: Large Melee, Melee, Magic Melee, Ranged, Gun, Shield, Strafing Shield, Magic Trinket, Trap, Consumable, and Miscellaneous.
Large Melee, Melee, and Magic Melee all have attacks and hitboxes that act similarly. Large Melee will generally "expend Focus" making them difficult for Magic users to use, though they also have high damage and stun, and are more likely to have Power and Armored Attacks. Regular Melee either postpone or have no effect on Focus Point generation. Magic Melee will follow these same rules, but if you have enough Focus Points, you may be able to cast while also attacking (e.g. the Drain Blade will cast Drain on itself and can steal HP on a hit).
Both Ranged and Guns use ammunition. Many of the ranged attacks will have an Aim mode that can be activated from any of the four command types. The Aim mode will lock your direction while allowing you to strafe.
There are regular Shields and Strafing Shields. Strafing Shields are a subset of shields that have flat faces and upon activation lock your direction. The rest of the Shields are curved and have the option of strafing like above or rotating like normal if activated while Tilted. Shields are further separated by other traits: Durability, Side or Back placement, Parry Strength, Walking Speed, Quick Turn, and Weight. Last note: when a Shield loses all Durability, pressing the Shield button will spend a moment to recover the shield instead.
Magic Trinkets and Magic Melee both can consume Focus Points to cast spells. Magic Trinkets will only have actions that are spells (and will sputter out when you don't have enough Focus Points). Magic Melee can do both, and will revert to their melee attack without a spell if you don't have enough FP.
Traps and Consumables both have limited uses. Traps are outwardly projected only e.g. Caltrops or the quickly exploding Drop Bomb. Consumables are generally used on your own person, though they can be thrown for varying effects e.g. the Magic Bottle can be ingested for a quick maxout for MP or it can be thrown for a small amount of splash damage.
The Miscellaneous category has stuff that doesn't fit nicely into categories above. The Goblin Head is like a Ranged Weapon, but it shoots fire. The Hook Chain is also shoots a projectile, but it is more about physics and doesn't do any damage. And the Dash Boots are also a little odd. These guys are the weirdos, and I think it helps keeping them away from the other items because they're so unique.