Item IDs, along with block IDs, are used with the "/give" command to allow an operator on a Minecraft server the ability to give any number of almost any item to a specific player. The command is given in the form: /give [player name] [Item ID] [quantity].
As of April 2015, lists of item IDs are available online on several Minecraft sites and forums, including Minecraft Information. This list is comprehensive and includes both basic blocks and complex crafted items such as horse armor and music disks. Codes are also listed for all the eggs from which users can spawn creatures and for all the potions used to fight and heal in the game.