App Indexing for Google Search

Websites have a lot way of getting it's content searched via search engine i.e google but mobile apps are not accessible for such search.

Smartphone users have tons of information on their phones, but the tools to find that information are rudimentary at best.

Your favorite restaurants, for example, might be stored in any number of apps, such as Foursquare or Yelp. Likewise, travel plans might be stored in an airline app or a travel app, such as Expedia or Orbitz. Finding that information requires first that you remember which app you used, since today’s search engines can’t penetrate the walls set up around each mobile application.

App Indexing allows you to connect pages from your website with specific content within your smartphone app. This enables smartphone users who have your app installed to open it directly from relevant mobile search results on Google.

Recently, a number of methods have emerged for finding information within applications. Such approaches are known as deep linking because they point not just to the right app, but to the specific place within an app’s structure where the information is stored.

App Indexing helps you drive usage of your app through Google. Deep links to your app appear in Google Search results on Android so users can get to your native mobile experience quickly and easily.

Follow this Google doc for implementing app indexing.

Apps with or without corresponding webpages can use the App Indexing API to notify Google of their deep links.

Along with Google, facebook is also providing API called App Links for app indexing.

Other companies are also providing their solutions, check below links:

http://appurl.org/
http://urx.com/

Link your app with Google Voice Search

Google with Version 3.5 has come extended it's voice search functionality to include device apps in its search result.

Now Activate Google voice search by saying "OK Google" from anywhere within your app or from anywhere within the device.

You are only required to add few lines of code in your manifest file.

<activity android:name=".SearchableActivity">
    <intent-filter>
        <action android:name="com.google.android.gms.actions.SEARCH_ACTION"/>
        <category android:name="android.intent.category.DEFAULT"/>
    </intent-filter>
</activity>

Once manifest changes are done, your app will receive the SEARCH_ACTION intent containing the SearchManager.QUERY extra with the search expression.

Assuming your app as MyApp. You can:
Activate Google: Say "OK Google"
Start Search: Say "search xyz on MyApp" or "Search for hotels in India on MyApp".

Inside your activity, you get the search string i.e xyz, hotels :
String query = getIntent().getExtras().getString(SearchManager.QUERY);

This feature is available on English locale Android devices running Jelly Bean and above with the Google app v3.5 or greater.

Check this OK Google from any screen and also check official post.