The following is a description of the various item types in the Kingdom of Loathing.
There are three types of consumable items: food and drink, booze, and miscellaneous.
Food and Drink
These are items you eat (or possibly drink) to gain more Adventures. Each food and drink item has an associated Fullness level; you will not be able to exceed your maximum Fullness level (usually 15). Liquid food items are termed beverages, and function in the same way as solid food.
Booze is alcoholic liquid, distinct from beverages. Drinking booze also gains you more Adventures. Each booze item has an associated Drunkenness level. Unlike food, you are able to exceed your maximum Drunkenness level, but not drink while falling-down drunk.
See also: Booze
The miscellaneous section contains various stat boosters and HP and MP restorers. Some of these items may have an impact on your spleen.
Hats are things you wear on your head, usually on top of it. All hats have a power rating, which determines how well they protect you from damage. Most hats have a minimum Moxie requirement to be equipped.
Back items help provide additional protection. The Back slot takes the place of the Container slot, and mostly covers backpacks, capes, shawls, and various shells.
See Back Items
Shirts are used to cover your torso. Most Adventurers aren't even aware they have a torso, so you need the skill Torso Awaregness to be able to equip shirts.
Weapons are what you use to hit your enemies. Weapons have a power rating determining their ability to deal damage. Most weapons have a minimum Muscle requirement to be equipped; some ranged weapons require a certain amount of Moxie.
Off-hand items are held in your hand which isn't holding a weapon. You may not equip an off-hand item if you are equipping a two- (or three-)handed weapon. Some off-hand items have a minimum Mysticality requirement to be equipped, while Shields, a specific type of off-hand item, may have a minimum Muscle requirement to be equipped.
Pants are used to cover your legs. All pants have a power rating, which determines how well they protect you from damage. Most pants have a minimum Moxie requirement to be equipped.
Accessories are various rings, amulets, gloves, shoes, watches and various other baubles you hang on various parts of your body. Accessories can provide a large variety of benefits. Most accessories have a minimum Mysticality requirement to be equipped. You may wear three accessories at one time.
See Off-hand Items
These are items needed to complete various quests, or in some cases, are quest rewards. All quest items are removed from inventory upon ascension. Some quest items are also equipment (weapons, accessories, etc) in which case they appear in that part of the inventory.
See also: Quest Items
(Mostly) Combat Items
These are items used mostly in combat. Some have other uses.
See also: Combat Items
These are items (which may or may not be potions) that give effects when used.
See also: Potions
These are items that don't fit into other categories. These items include familiar hatchlings, campground items, items that allow you into certain areas, ingredients, and several other things.