Here placed compact backpacks. They are created for caring small items, like warm clothes, documents and etc. Usually their volume under 20L. They working great during CQB.
Also knows as patrol backpacks or 3 day backpacks. Have a middle volume around 25-35 L. You can put everything what you need for 3 day trip in it - clothes, ration pack, and etc.
High capacity backpacks
If you planing to survive in Zombie Apocalypse and need everything for autonomous life - this category for you. Include big and VERY big backpacks (some of them can carry adult men inside). Some of this backpacks also have metal pins inside to support you back during long trips and reduce strain fatigue.
Haversacks usually used by Russian Units during long trip, when you need to put all your equipment in our bag. For just clothes take a look on small (up to 60 L) bags, for full set you need at least 60L haversacks. Very useful item, if you are lazy to to put things in a backpack each time.
Here you can find Special Backpacks, like medical one and cases for guns.