Android XML: android:elevation vs. app:elevation

When do I use android:elevation and when app:elevation?

What's the difference between those two?

Hope I can help, Let's talk with an example:

<android.support.design.widget.FloatingActionButton
    android:layout_height="wrap_content"
    android:layout_width="wrap_content"
    ...
    android:elevation="@dimen/elevation_medium"
    /> 

The android:elevationattribute will work from the API level 21 and upper.

<android.support.design.widget.FloatingActionButton
    android:layout_height="wrap_content"
    android:layout_width="wrap_content"
    ...
    app:elevation="@dimen/elevation_medium"
    /> 

In this case the app:elevation attribute belongs to the FloatingActionButton styleable, inside de Android Design Support Library which will work from version 4 of the framework, instead version 21, the case of android:elevation.