Selenium is a web testing automation framework for web applications and mobile website testing. Selenium is not designed for mobile application testing, but you could use a Selenium-based framework like Selendroid or Appium as they are created for automating mobile application testing.
Selendroid is a mobile testing tool used for only testing android applications whereas, Appium is preferred by a large section of testers which is used to test cross-platform applications in iOS as well as Android. Recently Selendroid comes bundled within Appium. So, you get the best of both platforms in a single framework.
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Also, do watch our Selenium Automation Tutorial video to get going.