Difference between Frame and Relative layout?

I'm new to android programming but from how much I have understood of the layouts from the documentation, RelativeLayout is mostly used when you need the views based on some rules and the FrameLayout when you want to overlap views.

But unfortunately for the following program I get the work of FrameLayout done by using RelativeLayout. I got my work done but for understanding, Am I missing something in the difference? Also, how did the buttons come over my image? (Even the other image is overlapping.)

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_parent" >

<ImageView
    android:id="@+id/imageView1"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="@drawable/ic_launcher"
    />

<ImageView
    android:id="@+id/imageView2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@drawable/ic_launcher"
    android:layout_alignParentTop="true"
    android:layout_alignLeft="@id/imageView1"
    />

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/imageView1"
    android:gravity="center"
    android:orientation="horizontal"
    android:weightSum="1.0" >

<Button
    android:id="@+id/button1"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_weight="0.33"
    android:text="Login" />

<Button
    android:id="@+id/button2"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_weight="0.33"
    android:text="Register" />

<Button
    android:id="@+id/button3"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_weight="0.33"
    android:text="Try application" />

</LinearLayout>

</RelativeLayout>