Junk items in Fallout Shelter are divided into seven types,[verification overdue] one for each tier of rarity; steel, cloth, leather, adhesive, circuitry, science, and valuables.
Junk items are obtained from lunchboxes, exploring the wasteland, found on quests, looted from the bodies of dead enemies. They can also be acquired by scrapping weapons and outfits, which will return junk items of the types that would be used to craft it in a workshop, but may also yield nothing.
They are used for crafting in the weapon workshop, outfit workshop, and theme workshop, with each weapon and outfit potentially taking up to three types of junk items in quantities of five, and themes taking up to five types of junk items in quantities of six.
There are a total of 21 (according to the Survival Guide as of version 1.13.13) available junk items.