The Minecraft Wiki Enchanting webpage lists a lot of mathmatical formulae and charts all designed to help us understand how enchanting works in Minecraft. I pretty much understand all of it.
Here's an excerpt for clarity:
The first thing that Minecraft does is apply two modifiers to the base enchantment level. Each modifier is restricted to a certain range, with numbers close to the middle of the range more common than those near the ends. The first modifier is based on the item's "enchantability," which depends on the material and the type of the item (see the table below). Minecraft picks a number between 0 and the enchantability, then adds that number plus one to the enchantment level.
Which keys off of this formula:
modified enchantment level = enchantment level + Random(0, enchantability) + 1
However, since the new addition of Bow Enchants in patch 1.1 I've become slightly grey on the idea of the enchantibility of a Bow, the table on the website does not give the enchantability for bows, however. Does anyone know it?
Edit: To clarify, from John in a comment below:
Enchantability is a special property of an item. It is a special number that helps choose which enchantment you get. So, he's asking what this special number is for bows, he's not asking what enchantments you can give a bow, or how high these enchantments go.