Backpack
A backpack (also called rucksack, knapsack, packsack, pack, Haversack, or Bergen) is a bag put on somebody's back.  It usually has two straps that go over the shoulders.  It is used to carry things in it, and it often has many pockets or compartments to carry things.  People often use backpacks when camping, hiking, or other activity where they need to carry many things.  Backpacks are also be used in the military by soldiers.  When used in school they are called bookbags or school bags.