Original: A backpack (also called rucksack, knapsack, packsack, pack, bookbag or Bergen) is, in its simplest form, a cloth sack carried on one's back and secured with two straps that go over the shoulders, but there can be exceptions.
Simplified: A backpack (also called rucksack, knapsack, packsack, pack, Haversack, or Bergen) is a cloth sack put on somebody's back.