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.
So, the name rucksack refers to a type of bag used by British people. The Americans, on the other hand, use the term backpack to refer to the same thing as the British.
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.