Items range from the mundane to the extraordinary. Offensive, defensive, and totally useless. Items can be used in a variety of ways depending on the stage, attacker, and target. Items are used by picking them up with the A button. After that depends on the item. Some you’ll throw at the target by tilting the control stick pressing A, some are activated by pressing A, some you merely pick up and they activate immediately. To drop or throw an item, press the Z (grab) button.
Items can further be broken down into 4 categories:
Normal (standard Smash Bros items)
Container Items (Items that hold other items)
Healing (heals the user)
Pokeballs (releases Pokemon)
http://www.supercheats.com/guides/super-smash-bros-brawl/assist-trophies (releases assist characters)
The items are listed in alphabetical order within their respective sections.