The major difference between onCreate() and onStart() is as follows.
onCreate() is called when the Activity is created; that is, it is launched or started. (Additionally, this triggers when the orientation is changed.) It is called again when the process is killed, then returned to.
while onStart() called following onCreate() at startup. Additionally, it is also called when the app is navigated back to after onStop() (and following onRestart()), which occurs after the Activity is no longer visible (which is during the time that "the user can see the activity on-screen, though it may not be in the foreground and interacting with the user").
