If it can go in your item pack or wharehouse, it is an item. However, this page will not cover weapons, units or disks. See the Weapons section for weapons, the Unit section for units, or the Photon Arts & Techniques sections for disks.
This hub will lead you to info on several types of items.
Basic Items: These show up in green and can be used. They will disapear after you use them. Some are recovery items or status boosters and others are special items.
Room Items: Anyone can own, store or sell these, but if you want to use them you must have access to "My Room", which is a premium item that must be purchased with real money via the AC Cash system. These are cool items you can use to decorate your room. Some are things to look at, but others can be used, sat on, etc. There are alot of fun items in this catagory.
Quest Items: Many of the NPC characters offer quests, and some of thoes quests involve collecting unusual items. Any of these items will appear under the quest item tab, often marked with icons of boxes, claws or feathers. Save up these items to complete quests. Once you complete the quest, they will no longer drop and when you go to collect your reward for the quest, you will automaticly turn in the required items. You can sell left over quest items to the NPC shops, but they are mostly worthless there. You can also buy and sell them in the player stores where the minimum price is 1000 mesets plus tax. (yes there is tax, lol) Other than this, these items have no use.
Misc. Items: This catagory has all the ods and ends. Icons are purple normally and they get their own tab in your inventory too. There are items as simple and common as grinders or as unique and exotic as redecorating tickets and mag devices.