ReactJS is a popular JavaScript library for building user interfaces, particularly single-page applications. Developed by Facebook, React allows developers to create reusable UI components, making it easier to build dynamic and interactive applications. Its component-based architecture helps in organizing code and improving maintainability, as each component can be independently updated without affecting the entire page.

React uses a virtual DOM to optimize rendering, making it highly efficient in updating and displaying changes on the screen. With its focus on performance, scalability, and developer productivity, React has become the go-to solution for building modern web applications. It is often paired with state management libraries like Redux to manage data flow and application state.