React.js is a popular JavaScript library developed and maintained by Facebook. It is primarily used for building user interfaces for single-page applications. React.js follows a component-based architecture, allowing developers to break down the UI into reusable components. It uses a virtual DOM (Document Object Model) for efficient rendering and updates only the necessary components when the data changes.
React.js provides a declarative syntax, making it easier to describe how your UI should look at any given point in time. It uses JSX (JavaScript XML) to combine HTML-like syntax with JavaScript, allowing you to write expressive and reusable UI components. React.js also supports server-side rendering, making it suitable for developing isomorphic applications.