The item editor, while only having one screen, is easily as complex as the Enemy editor.
 
 
Binoculars: Runs a case-sensitive search through the item names.
123 Button: Sorts the item list by order in the ROM.
ABC Button: Sorts the item list alphabetically.
Arrow Button: Sorts the item list by type.  (Food, weapons, tools...)

Item:  Just a dropdown selection menu.
Name: This lets you type in the name of the item.  Careful, if it's too long you won't be able to select things in your menu properly.  A good rule of thumb is to keep it shorter than or equal to "Cup of Lifenoodles," taking off a letter for each extra capital letter.  Another interesting thing to note is that the | (Shift + \) character means the musical note in an item name.
Effect: What the item does.  Careful, some effects are meant for only in-battle use, or only for out of battle use.
Cost: How much it costs to buy the item in a store.  You can sell items for half that price.

Type: Tells the game what to treat the item as.  From Tomato's source code, here is a list of types.
16 = Weapon (Bash in Menu) 17 = Weapon (Shoot in Menu) 20 = Body equipment
24 = Arm Equipment 28 = Other Equipment 40 = Condiment
32 = Food 36 = Drink 0 = Only used by the Franklin Badge!?
4 = Teddy Bear 8 = Broken Item 44 = Large Pizza
48 = Can be used in or out of battle. 52 = In-Battle Only Item 53 = Fly Honey, Dragonite?
56 = Out of Battle Only Item 58 = Location - Dependent Item 59 = Important Plot Item

Strength, Extra Power, and Extra Power Increase: I've come to the conclusion that when the guys at Ape coded this part of the game they had been hitting the sake pretty hard.  These functions seem to interleave among different types of items.
-- In weapons and armor, strength is the offense/defense modifier.
-- In broken items, EP and EPI seem to be the item they repair to, interchangably, and sometimes not at all.
-- Sometimes EPI seems to be a bonus modifier of sorts when you equip the item.
-- In some items like the various capsules, strength seems to indicate a particular stat to modify.
I hate these things.

Special Property:  This seems to be related to specific defenses for equipped items. This is a small, unconfirmed list:
  2 = 50% Fire Protection
  3 = Full Fire Protection
  8 = 50% Ice Protection
  12 = Full Ice Protection
  32 = 50% Flash Protection
  48 = Full Flash Protection
  128 = 50% Paralysis Protection
  192 = Full Paralysis Protection

Description Address: Indicates where to look in the ROM for the "Help!" description.
Who can use?: I think that's pretty obvious as it is.
Infinite use: When you use it, it doesn't leave your inventory.  A permanent item will vanish unless it has infinite uses.
Permanent: You can't sell it, drop it, get it stolen, or give it away.  Only a code block with the "Take an item" code will move it.
Chicken: Eventually, this will become item 168.  Then, if it's checked as a chicken, it will become item 169.

Note: I'm angry, because I typed this up then saved over it.  So, wah.