React is the most powerful JavaScript library, which is used for building user interfaces. It was developed by Facebook and is often used for building single-page applications and mobile applications. React was released first in 2013, and since then, it has become one of the most popular JavaScript libraries for building user interfaces. It was developed by Jordan Walke, a software engineer at Facebook, and was initially released as an open-source project on GitHub. 

One of the main features of React is its virtual DOM (Document Object Model), which allows it to update the user interface when the underlying data changes efficiently. This makes it a good choice for building dynamic and interactive user interfaces. React has also been widely adopted by the developer community, and there are many third-party libraries and tools available that integrate with React. In addition, Facebook has released a number of other related tools and libraries, such as React Native, which allows developers to build native mobile applications using React. 

What exactly is React JS: 
React is a JavaScript library that is used to build user interfaces, particularly for single-page applications. It was developed by Facebook and is often used for building web applications that are fast, scalable, and maintainable. React allows developers to create reusable UI components, which can help make building and maintaining a user interface easier and more efficient. Instead of having to rewrite the same code every time you want to create a similar element on a webpage, you can simply use a React component you've already built. 

One of the key benefits of React is that it allows developers to declaratively describe their user interfaces, which can help make the code easier to understand and debug. Additionally, React uses a virtual DOM (a lightweight in-memory representation of the actual DOM) to optimize updates to the actual DOM, which can help improve the performance of applications that rely on frequent updates. 
React also has a strong ecosystem of tools and libraries that can be used to build, test, and deploy applications. For example, React Router is a popular library for adding routing to React apps, and Redux is a popular library for managing application state.