Selenium is a automation tool used for WEB Application Testing & it is a opensource tool. We can start right away by downloading & installing. Few test process can be performed i.e - GUI Testing, E2E Testing, Functional Testing, Sanity Testing, Cross Browsing Testing, Regression Testing & Usability Testing. It is designed to test Websites & Web Apps.
Appium is also a automation tool used to test Mobile Applications. The platforms include either Android / iOS. We can connect directly our mobile to the system and run scripts. where it create scripts automatically. It aslo support all supports all programming languages along with selenium libraries. It is designed to test mobile apps. It uses selenium API to test the applications.
Conclusion: Having knowledge in both the platforms leads to several oppurchinities into Testing. Learn Appium, Selenium with newest updates such that you career growth can be upscale.