React is a popular open-source JavaScript library used for building user interfaces, particularly for single-page applications. Developed and maintained by Facebook, React allows developers to create reusable UI components, making it easier to build and manage complex user interfaces.

Key features of React include:

1. Component-Based Architecture: React applications are built using components, which are self-contained, reusable pieces of code that define how a part of the UI should appear and behave. This modularity improves maintainability and reusability.

2. Virtual DOM: React uses a virtual DOM to optimize updates to the actual DOM. When the state of a component changes, React creates a virtual representation of the DOM, compares it with the previous version, and efficiently updates only the changed parts in the real DOM. This process, called reconciliation, enhances performance.

3. JSX: JSX is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript. It makes the code more readable and easier to write. JSX is then transpiled to JavaScript by tools like Babel.

4. Unidirectional Data Flow: React follows a unidirectional data flow, meaning data flows in one direction from parent components to child components. This makes it easier to understand how data changes affect the UI.

5. Hooks: Introduced in React 16.8, hooks are functions that let developers use state and other React features in functional components. Hooks like useState, useEffect, and useContext simplify state management and side effects in functional components.

6. Ecosystem: React has a rich ecosystem with various libraries and tools that complement it, such as React Router for routing, Redux for state management, and Next.js for server-side rendering.

React is widely used in the industry for building dynamic, high-performance web applications. Its declarative approach and powerful features make it a preferred choice for developers working on both small and large-scale projects.