React is an open-source JavaScript library developed by Meta (formerly Facebook) for building dynamic and interactive user interfaces. It allows developers to create reusable UI components that manage their own state. Instead of directly manipulating the browser's Document Object Model (DOM), React uses a virtual DOM, which is a lightweight representation of the actual DOM. When component states change, React first updates the virtual DOM, then efficiently calculates the minimal changes needed and applies them to the real DOM, significantly improving performance and simplifying UI updates. This script provides the core functionality of the React library, enabling web applications to render components, manage application state, handle events, and create a responsive user experience. It serves as the foundation for single-page applications (SPAs) and complex web interfaces, making development more modular and maintainable.