From Diablo Wiki
Junk is, technically, an item type. Junk refers to items that have no value to the player, perceived or otherwise, such as getting a town portal scroll when you have an entire tome full, or a quiver of arrows dropping when you don't use a ranged weapon. These items would be referred to as junk.
Junk can often also be called "vendor trash" which are items that the player picks up solely for the purpose of selling to a vendor. Rings, Amulets, and class-specific items were often vendor trash in Diablo II, but the player would pick them up simply because they took up very little inventory space, and tended to be worth more than the average item (in terms of gold, at any rate).