A rucksack is a bag you carry on your back, with straps over both shoulders. The picture in most British minds is a bag for a long walk, a hill climb, or a weekend away — not a tiny fashion backpack, though people do stretch the word. Americans more often say backpack; rucksack sounds a bit more outdoor, or a bit more British, and the army still likes it for a big field pack. Though actually, in a shop, the two words often point at the same kind of thing. It is an ordinary, practical word, not fancy. You pack a rucksack, sling it on, hike with it. It is not a suitcase, and it is not a bag you hold in one hand.