Android layout xml: difference "app:" and "<view_name>:"

Using either of this prefixes gives the same result:

    app:cardElevation="4dp"
    card_view:cardElevation="4dp"

Are these only synonyms? And if so, why do both variants exist?

EDIT

As Doomsknight said in the comments it only happens to do the same when the namespace is exactly the same (which makes sense).

However I still don't understand where the difference between those prefixes lies.