I\'m writing my third app, and I already have an app in the App Store, but I still don\'t get this App ID business.
I created the App IDs for my first two applicatio
Both are "proper", it's mostly a question of the tradeoff between ease of management and allowing more granular management of certificates.
There two technical difference:
Applications with IDs that are identical save from the top level (e.g. com.mycompany.aaa and com.mycompany.bbb) are able to access each others saved data. If you are writing a suite of applications then this could be extremely useful.
You can only create a limited number of "AdHoc" certificates. If your apps have the same wildcard identifier then this limit would be imposed across all your applications.
Personally for a small developer I would recommend using a wildcard ID. It results in a lot less to manage in several areas, and hence removes a lot of potential for mistakes. Remember that for each ID you'll probably have three individual certificates (dev/adhoc/appstore).
Background:
Every iPhone application must have a unique identifier and certificate. Developers can either create a certificate per application by using a complete name (e.g. com.mycompany.aaa), or they can create a wildcard certificate/ID (e.g. com.mycompany.*) in which case the app name in the .plist file is used to complete the identifier during the DRM process.