Android : What is the difference between activity and fragment

My research ::Significant difference is the concept of backstack and how they exist

Activity::

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  When an activity is placed to the backstack of activities the user can navigate back to the previous activity by just pressing the back button.

  
  


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  Activity can exist independently.

  
  

Fragment::

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  When an fragment is placed to the activity we have to request the instance to be saved by calling addToBackstack() during the fragment transaction.

  
  


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  Fragment has to live inside the activity

  
  

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Are there any additional differences ?