I got tired of wondering what each bag looked like and how much they could hold so here they are. Bag variant Class Names are formed like this: US_Assault_Pack_EP1
for example. The whole list can be found on the wiki (http://community.bistudio.com/wiki/ArmA_2_OA:_Backpacks). Images of all the various bags in game can be found on ArMaTeC's wonderful library of objects
These bags can't hold cargo, but can be used for things like making into a UAV controller or creating crew served weapons.