Appium is used for automated testing applications on a mobile device, including mobile application versions, mobile browser application versions, and Hybrid. On the other hand, Selenium is used for test automation of web applications and does not support system & mobile applications.
Both Appium and Selenium are open source and available for free of cost. Selenium supports multiple browser settings and operating systems. Similarly, Appium supports all the leading mobile system software like iOS, Android and Microsoft Windows.
Source: DridhOn