A backpack, also called knapsack, schoolbag, rucksack, pack, booksack, bookbag, haversack, packsack, or backsack, is in its simplest frameless form,a fabric sack carried on one's back and secured with two straps that go over the shoulders, and is used to carry goods from one place to another, A bag isa lightweight, flexible container meant for carrying things.In some places the word sack is more common, but the handled container you get at the store when you buy something is usually called a bag.