React is a popular JavaScript library for building user interfaces, particularly for web applications with dynamic content. Developed by Facebook, React allows developers to create reusable UI components and manage the state of those components efficiently. It follows a component-based architecture, where the user interface is divided into independent and reusable components, making it easier to manage complex UIs.

Key features of React include:

1. Virtual DOM: React uses a virtual DOM (Document Object Model) to efficiently update the UI. Instead of directly manipulating the DOM, React creates a lightweight virtual representation of it in memory, compares it with the previous state, and applies only the necessary changes to the actual DOM. This helps in improving performance and responsiveness.

2. Component-Based Architecture: React applications are built using components, which are reusable and independent building blocks of the user interface. Components encapsulate both the structure and behavior, making it easier to develop and maintain large-scale applications.

3. JSX: React uses JSX (JavaScript XML), which is an extension of JavaScript syntax that allows developers to write HTML-like code within JavaScript. JSX makes the code more readable and intuitive, especially when defining the structure of UI components.

4. Unidirectional Data Flow: React follows a unidirectional data flow, where data flows in a single direction from parent components to child components. This helps in maintaining a predictable state of the application and makes debugging easier.

5. Declarative Syntax: React promotes a declarative programming style, where developers describe what the UI should look like at any given point in time, and React takes care of updating the DOM to match that state. This approach simplifies the process of building complex UIs and reduces the likelihood of bugs.

6. React Hooks: Introduced in React 16.8, Hooks are functions that allow developers to use state and other React features in functional components without writing a class. Hooks enable better code reuse, composition, and logic organization in functional components.

React is widely used in the development of single-page applications (SPAs), progressive web apps (PWAs), and mobile apps using frameworks like React Native. Its popularity stems from its performance, flexibility, and robust ecosystem of libraries and tools, making it a preferred choice for front-end development in many organizations.