There are separate skills for bows and crossbows - you get to improve your accuracy, damage, reload time etc for the weapon of your choice.
I never found a bow or crossbow that did nearly as well for me as a melee weapon so I never stayed an archer very long. Melee weapons can be sharpened but not missile weapons. Crossbows seemed to do more damage but maybe later in the game bows were better or maybe it was just the luck of the drop.
There are also skills you can select making you better with specified types of arrows.
In terms of attributes you will need both strength and agility to qualify for the better bows abd crossbows - in my limited experience strength is more for crossbows.
