As I've played around with different ROMs and used Titanium Backup, I see that apps are categorized into system and user apps. Several ROM developers state that Titanium Backup should only be used for user apps and not for system apps, and when I proceed to uninstall system apps, TiBu warns me that the ROM may not work correctly.
Other than the fact that user apps are downloadable via the Market or some other means, what are the key differences between the two?
What is it about the system apps that make them more integral to the OS itself?
Answer
/system is read-only without root so it prevents uninstalling applications from /system/app and /system/priv-app. Applications that are critical are put there so that they can't be uninstalled. Carrier bloatware that they don't want to let you remove is also put there. The latter category of apps can be removed if you are rooted; the former, not so much.
/system/priv-app also allows apps installed there to use signatureOrSystem and other privileged permissions. Stephen's answer has some examples.
As for why apps might be critical, that depends on the ROM. HTC's Sense is heavily dependent on their built-in apps from what I hear, for example. They simply didn't design their UI such that it would work natively with third party apps (or fail nicely if the system apps were missing). Other apps are essentially part of the Android OS but Google coded them as apps/services (probably for encapsulation and reliability reasons).
Updates for some system apps can be downloaded via the Play Store, such as Google Maps. If it didn't come preinstalled as a system app on your device then you can use Titanium to convert it from a user app to a system app.
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