The difference is that fragments have a life cycle (onPause, onCreate, onStart...) of their own. By having a life cycle, fragments can respond independently to events, save their state through onSaveInstanceState, and be brought back (i.e. such as when resuming after an incoming call or when the user clicks the back button). The life cycle is summarized in the fragment documentation:
https://developer.android.com/guide/components/fragments.html#Lifecycle
You can always wrap a fragment or activity around a compound view, so just think of fragments as containers to your compound views that give them an independent life cycle.