Selenium: Selenium is an open-source (free) automated testing framework for validating web applications across multiple browsers and platforms. Selenium Test SCRIPTS can be written in a variety of programming languages, including Java, C#, Python, and OTHERS. 

Following are the differences between Appium and Selenium:-

Based on USAGE:

• Appium - Appium is a popular open-source for automation testing, notably in native applications such as Android and iOS apps. It can ALSO be used in hybrid applications that run in both Android and iOS environments. This type of automation testing assures that no systemic issues arise as a result of any specific patch installation once the app has gone live in production. It assures complete integration testing and avoids a significant amount of manual labour on the TESTER's part. By using Appium, users can easily avoid manually testing each and every feature of the entire application every time.
• Selenium - Selenium is a common automated testing tool that is built for any type of web application. It is compatible with all current popular browsers and operating systems such as Windows, Linux, and MACOS.

Based on Design:

• Appium - Appium is primarily intended as an HTTP server because it will handle any type of mobile app. However, it is primarily following or developing the same in node JS, rather than utilising standard Java or JS code. As a result, developers who want to utilise Appium for automated testing in any type of mobile app must first install Node JS on their system before using the Appium tool.
• Selenium - Selenium is primarily used for maintaining automation logic on web applications or webpages. It is created in such a way that it may interact with the application quickly and easily by using browser activities.