(* backpack )
I think they are the same... The former one is English and the latter comes from German (Rücken = back, Sack = bag, sack), otherwise no difference...
I did some googling and I now think that "rucksack" is something like a туристичний рюкзак (those really big ones)
I'm English, and would say rucksack and backpack are fairly interchangeable, but I'd be more likely to use backpack for a big one for holidays or expeditions, and rucksack for a smaller, everyday one.