As I understand it, an App ID is an code that's unique for an single application. Every time I start developing an new application, I have to create a new App ID.
But I don't get it what's up with that "provisioning profile". Do I need one for each single app? What's that for?
It's to protect the end users, by having a way to validate apps and their origin. It also serves as a centralized system to control development. On windows most apps use GUIDs for this, but they don't have any centralized info about them. Apple does, which is a pain at times, but it's a valid effort in the end.
The provisioning profile contains a list of devices that are allowed to run or debug your code. Everything is verified with a digital signature, which ultimately lets Apple control distribution. This is why you need to jailbreak in order to get apps from sources outside the App Store.