how android recognize the difference between built in apps and third party apps

how android recognize the difference between built in apps and third party apps. whats are the measures do it use to recognize. 开发者_如何学Cthanks.

An app is recognized by the package it uses. All built-in apps have a common starting package name (Eg: com.android.... or com.htc...) not sure how identification between apps is of application :P