An Inventory is an object that contains other things. It is used to track what items are in a container, or which items a Person carries. A different specialized inventory is used to track what apparel a person is wearing (see SmutBook.Apparel).
A trunk or bookshelf might use an inventory to track what is held within or upon it. In a more traditional sort of location-based Twine IF, each location might even use an inventory to track what items the player can find or has dropped there.
GeneralInventory is a baseline inventory container object that can be used as-is, or subclassed to implement inventories that behave differently (as is done with Wardrobe and WornApparel).
InventoryUI is a tool to render an inventory manipulation GUI into a jQuery DOM element, to allow the player to move things between inventories (ie take or drop items). It can also be subclassed to implement InventoryUIs that behave differently (as is done with ApparelUI, the interface for wearing and removing clothing).