I have read about the toolbar coming in the AppCompat Library and all its features. Some of the things I have noticed in android developers blog (here) is:
Toolbar is fully supported in AppCompat and has feature and API parity
with the framework widget.
<include layout="@layout/content_myactivity" />
AppBarLayout is a container, where you can place
TabLayout, something else maybe. All of it will be shown on top of the screen, no matter what layout you use for the rest of the content. You can use
AppBarLayout if you want, but then you'll need to include it in your
ViewGroup which you use the rest of the content. And place it on the bottom of it, so it won't get overlayed with something else.
AppBar saves you from that and provides some additional features, for instance, scrolling behavior. It is written here btw.
Also note, that
AppBarLayout depends heavily on being used as a direct child within a
CoordinatorLayout. If you use
AppBarLayout within a different
ViewGroup, most of it's functionality will not work.