A backpack (also called rucksack, knapsack, packsack, pack, Haversack, or Bergen) is a cloth sack 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 compartments to carry things. People often use backpacks on camping trips, hikes, or any form of outdoor activity where people need to carry many things. Backpacks are also be used in the military by soldiers. It can be also used in school, or in this case, it also called a bookbag or school bag.