Differnce between abilities of System apps, Apps in phone memory and Apps in the SD card Possible Duplicate:
  What are the differences between a system app and user app?  




What is the difference between abilities of System apps, Apps in phone memory and that in the memory card?

The only difference that I know about currently is that Widgets' of Apps in the memory card do not show up in the widgets' list and thus can't be used?

Are there any more differences? What are they and why?
Definitions


System App: an app that is installed on the (usually read-only) System partition (/system)
User App: all others (installed in phone memory or on the internal/external sdcard)


User-Apps and System-Apps

Concerning System-Apps versus User-Apps, please see What are the differences between a system app and user app? as suggested by dymutaos.

Apps on SD

A special case are "Apps on SD". Starting with Android 2.2 (aka Fozen Yoghurt or, in short, Froyo), App2SD became a standard feature to the Android system. However, not all apps can be installed/moved to SD card this way, and for good reasons:


the developer might decide to not permit it (see App Install Location in the Android developer documentation)
certain functionalities do not permit it, as they would no longer work


In the second group belong...


AutoStart: Apps could no longer be automatically started by the system (at boot_completed), as the SD card is only mounted after that event was broadcast
when mounting the sdcard to a computer as USB storage, it is no longer available locally on the device -- hence the apps are no longer available -- hence they would be closed. Quoting from the mentioned source:  


  When the user enables USB mass storage to share files with a computer or unmounts the SD card via the system settings, the external storage is unmounted from the device and all applications running on the external storage are immediately killed.

services offered by such apps would not be available because of the above
Widgets will fail exactly because of the above reasons
same issues for input method engines,  live wallpapers, account-managers, sync-adapters, and more (all described on the above linked source)


Sure there are possibilities to force almost any app to SD (e.g. DroidSail has this ability) -- but that would break above mentioned features, if any such app offers them. Another alternative is offered by e.g. Link2SD (rooted devices only), which utilizes a separate partition for those apps -- a partition not mounted to the PC, so a few restrictions are circumvented.

Summary

Basically, there's no difference between the apps -- but it makes a difference where they are installed. Certain privileges are reserved to system apps only, as explained in this answer -- but system apps cannot be moved to sd. Apps on SD have certain restrictions, so not every app can be moved there -- but the good of it is: if an app can be moved there, it might save you valuable space on the internal storage, especially when that is limited (as it often is on low-end devices).