All items have a category. For a complete list of items sorted by category, see Category:Item. Items are also organized in four sections in a character's inventory. For a list of each section's categories, see the pages for Equipment, Miscellaneous, Resources and Quest Items.
There are also some items that don't show up in the inventory at all. They usually appear as icons at the top of the screen, and are items in name only. See effect item for more details.