Armory is a transmog mod that allows you to wear one item in a given slot, getting any stats/bonuses/abilities it gives, while showing the appearance of another item on your character.
I haven't used other mods like Transmog Enhanced, so I can’t compare, but from what I’ve gathered, Armory is more slot based than some transmog mods.
With Armory, you set the appearance of a specific slot for a character, and it uses that appearance even if you change the item equipped in that slot. The transmogged appearance is saved for the slot, not for a specific item. So you’re not setting Astarion’s starting armor to be transmogged to skimpy leather armor, you’re setting whatever is equipped in his body armor slot to be transmogged to skimpy leather armor.
Another feature of Armory is that you can use any item in any slot, no matter what slot it’s coded to use. You can set your camp footwear slot to show a hat or your helmet slot to show modular extra armor pieces and so on.
The mod page for Armory is a little intimidating - I was hesitant to try it for a long time! And while it's not actually as complicated as it looks, it’s not the most intuitive to figure out at first, hence this page.