APPIUM is a freely distributed open source mobile application UI testing framework.
Appium allows native, hybrid and web application testing and supports automation test on physical devices as well as on emulator or simulator both.
It offers cross-platform application testing i.e. single API works for both Android and iOS platform test scripts.
It has NO dependency on Mobile device OS. Because, APPIUM has framework or wrapper that translate Selenium Webdriver commands into UIAutomation (iOS) or UIAutomator (Android) commands depending on the device type not any OS type.
Appium supports all languages that have Selenium client libraries like- Java, Objective-C, JavaScript with node.js, PHP, Ruby, Python, C# etc.