Everyone I know calls backpacks and bookbags interchangeably, with a few people saying knapsack (that one I never understood the root of, but its a normal enough word here). I had a friend from the Quad cities (Illinios) who called it a rucksack, never heard anyone else say that!
In general anything can be a bag: a grocery bag, a backpack isa bag, a purse is a bag, a garbage bag is a bag, a tote is a bag, etc. But a "sack", unless part of a compound word, in my experience is a shapeless bag like Santa's sack or a potato sack, or using a pillowcase as a Halloween sack.