What is the difference between android:layout_alignRight="ID" and android:layout_toRightOf="ID"?

This picture will explain the difference :

layout_alignRight aligns the right edges of the two views.

layout_toRightOf aligns the left edge of this view with the right edge of the other someId view.

Documentation: http://developer.android.com/reference/android/widget/RelativeLayout.LayoutParams.html