Backpacks are unlocked in the Leather collection, and let you interact with them to open and store items inside. It can be opened by right clicking while in your main hand, but you can also open it by right clicking on it when it is in any slot of your inventory, ender chest, etc. Backpacks can also be dyed to any color you would like.
There is an Uncommon version of a backpack that you can place, but it has no use and just serves as a trophy. However, it is unknown how to get this backpack as only 32 exist. This placeable version of a backpack only comes in purple.[Source needed]
A backpack cannot be placed in another backpack or sold to an NPC, unless it's one of the placeable backpacks mentioned before.
Backpacks are notorious items used in Scams by hiding junk items inside and trading them away as something else. A hidden update on May 27, 2020 revealed that item names were stored inside a backpack, making backpack scams less effective; however, this change was reverted.