React is an open source library for building interfaces. ReactJS is what allows developers to create declarative UI on the web more easily while React Native lets you leverage your skills for native mobile platforms like iOS and Android.
I've worked with React for many years professionally and it's been a blast. React is the library that's flexible and fun to use. It's been my most relied upon tool in web development in my career due to its large community and ease of integrating with projects. Because of its popularity, React has lead to some other really good frameworks for building whole web apps like Next.js.
If you're looking for a library for building user interfaces on the web that has a large and vibrant community then look no further than React.