Is system apps and pre-installed apps two different concepts? I recently started learning android ROMs. I have heard system apps with some special (not root) permissions.

Is there a difference between system apps and pre-installed app when we build a ROM? I those two concepts?
I think I might have been found the difference. According to this link :


  If an app is signed with System Signature, then its not allowed to
  Disable that app.Also , not all preloaded apps have system signature.


So among the apps within /system/app directory, we have two types of pre-installed apps. System signed with more privileges and normal apps.