As an open platform, Android offers choice. You distribute your Android apps to users in any way you want, using any distribution approach or combination of approaches that meets your needs. From publishing in an app marketplace to serving your apps from a web site or emailing them directly users, you are never locked into any particular distribution platform.
The process for building and packaging your app for distribution is the same, regardless of how you will distribute your app. This saves you time and lets you automate parts of the process as needed. You can read Preparing for Release for more information.
The sections below highlight some of the alternatives for distributing your apps to users.
Distributing through an App Marketplace
Usually, to reach the broadest possible audience, you would distribute your apps through a marketplace, such as Google Play.
Google Play is the premier marketplace for Android apps and is particularly useful if you want to distribute your applications to a large global audience. However, you can distribute your apps through any app marketplace you want or you can use multiple marketplaces.
Distributing your application through email
The easiest and quickest way to release your application is to send it to users through email. To do this, you prepare your application for release and then attach it to an email and send it to a user. When users open your email message on their Android-powered device, the Android system will recognize the APK and display an Install Now button in the email message (see figure 1). Users can install your application by touching the button.
Note: The Install Now button shown in Figure 1 appears only if users have configured their device to allow installation from unknown sources and have opened your email with the native Gmail application.
Distributing applications through email is convenient if you are sending your application to only a few trusted users, but it provides few protections from piracy and unauthorized distribution; that is, anyone you send your application to can simply forward it to someone else.
Distributing through a web site
If you do not want to release your app on a marketplace like Google Play, you can make the app available for download on your own website or server, including on a private or enterprise server. To do this, you must first prepare your application for release in the normal way. Then all you need to do is host the release-ready APK file on your website and provide a download link to users.
When users browse to the download link from their Android-powered devices, the file is downloaded and Android system automatically starts installing it on the device. However, the installation process will start automatically only if users have configured their Settings to allow the installation of apps from unknown sources.
Although it is relatively easy to release your application on your own website, it can be inefficient. For example, if you want to monetize your application you will have to process and track all financial transactions yourself and you will not be able to use Google Play's In-app Billing service to sell in-app products. In addition, you will not be able to use the Licensing service to help prevent unauthorized installation and use of your application.
User Opt-In for Apps from Unknown Sources
Android protects users from inadvertent download and install of apps from locations other than Google Play (which is trusted). It blocks such installs until the user opts-in Unknown sources in Settings > Security, shown in Figure 2. To allow the installation of applications from other sources, users need to enable the Unknown sources setting on their devices, and they need to make this configuration change before they download your application to their devices.
Note that some network providers do not allow users to install applications from unknown sources.