The action bar is one of the most important design elements we can implement for our app’s activities .It provides several user interface features that make our app immediately familiar to users by offering consistency between other Android apps. Key functions include:

A dedicated space for giving your app an identity and indicating the user’s location in the app.

Access to important actions in a predictable way (such as Search)

Support for navigation and view switching (with tabs or drop-down lists).

Setting up the Action Bar:

In its most basic form, the action bar displays the title for the activity and the app icon on the left. Even in this simple form, the action bar is useful for all activities to inform users about where they are and to maintain a consistent identity for our app.

Setting up a basic action bar requires that your app use an activity theme that enables the action bar. How to request such a theme depends on which version of Android is the lowest supported by your app

Beginning with Android 3.0 (API level 11), the action bar is included in all activities that use the Theme.Holo theme (or one of its descendants), which is the default theme when either the targetSdkVersion or minSdkVersion attribute is set to "11" or greater.

Adding Action Buttons

The action bar allows us to add buttons for the most important actions items relating to the app’s current context. Those that appears directly in the action bar with an icon and/or text are known as action buttons. Actions that can’t fit in the actions or aren’t important enough are hidden in the action overflow

Specify the Actions in XML

All action buttons and other items available in the action overflow are defined in an XML menu resource. To add actions to the action bar, create a new XML file in the project’s re/menu/ directory.

<menuxmlns:android="http://schemas.android.com/apk/res/android">
    <!-- Search, should appear as action button -->
    <itemandroid:id="@+id/action_search"
          android:icon="@drawable/ic_action_search"
          android:title="@string/action_search"
          android:showAsAction="ifRoom"/>
    <!-- Settings, should always be in the overflow -->
    <itemandroid:id="@+id/action_settings"
          android:title="@string/action_settings"
          android:showAsAction="never"/>
</menu>

This declares that the Search actions appear as an action button when room is available  in the action bar , but the settings  action should always appear in the overflow.(By default, all actions appears in the overflow, but its good practice to explicitly declare them

The icon attributes requires a resource id for the image. The name that follows @drawable/ must be the name of a bitmap image you've saved in your project's res/drawable/ directory. For example, "@drawable/ic_action_search" refers to ic_action_search.png. Likewise, the title attribute uses a string resource that's defined by an XML file in your project's res/values/ directory

Add the Actions to the Action Bar 

To place the menu items into the action bar, implement the onCreateOptionsMenu() callback method in your activity to inflate the menu resource into the given Menu object. For example:

Respond to Action Buttons

When the user presses one of the action buttons or another item in the action overflow, the system calls your activity's onOptionsItemSelected() callback method. In our implementation of this method, call getItemId() on the given MenuItem to determine which item was pressed—the returned ID matches the value you declared in the corresponding <item> element's android:id attribute.

Add Up Button for Low-level Activities

All screens in your app that are not the main entrance to your app (activities that are not the "home" screen) should offer the user a way to navigate to the logical parent screen in the app's hierarchy by pressing the Up button in the action bar.

When running on Android 4.1 (API level 16) or higher, or when using ActionBarActivity from the Support Library, performing Up navigation simply requires that you declare the parent activity in the manifest file and enable the Up button for the action bar.

For example, here's how you can declare an activity's parent in the manifest:

Then enable the app icon as the Up button by calling setDisplayHomeAsUpEnabled():

Because the system now knows MainActivity is the parent activity for DisplayMessageActivity, when the user presses the Up button, the system navigates to the parent activity as appropriate—you do not need to handle the Up button's event.

This post is referred from Google docs  Adding the Action Bar