Appium is a free, open-source test automation framework for mobile testing that allows developers to write tests using any language supported by Selenium WebDriver. It provides a standardized JSON over HTTP protocol and can automate native, web, and hybrid mobile apps on real devices, simulators, or emulators. The Appium server communicates with client libraries tailored to each language, enabling code reuse between test suites. Testing in the Appium framework is designed for mobile testing only and takes advantage of developer skills, allowing for automated testing of GUI-related widgets and controls. It integrates with continuous integration servers and supports various platforms such as iOS and Android, while following philosophical tenets of openness, flexibility, and avoiding unnecessary recompilation or modification of apps.