Linear and Relative

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

        <WebView
            android:id="@+id/webView1"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

        <com.google.android.gms.ads.AdView
            android:id="@+id/adView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:layout_alignParentLeft="true"
            ads:adSize="BANNER"
            ads:adUnitId="xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx" >
        </com.google.android.gms.ads.AdView>

    </RelativeLayout>

</LinearLayout>