The word 'rucksack' comes from Germany and the term is frequently used in Western military forces, such as those in the UK and US. 'Rucking' is one of the most common military activities and so it's clear to see how rucksacks get their name.
A backpack is a bag worn on your back and typically has two straps over the shoulders. A rucksack is a specific backpack most often used for hiking or camping. It is larger than a backpack and usually has a frame to support the bag's weight.
In British English, the same kind of bag is called a sports bag. A holdall (or occasionally hold-all) may be a similar bag but may often have wheels and possibly a telescopic handle.
Carrying a backpack instead of a purse has massive benefits. My right shoulder is higher and tighter than my left shoulder from years of purse carrying. Wearing a backpack has not only prevented further asymmetry, but I am starting to see my shoulders even out.
A randoseru (?????) is a firm-sided backpack made of stitched firm leather or leather-like synthetic material, most commonly used in Japan by elementary schoolchildren.