React, also known as React.js or ReactJS, is an open-source JavaScript library for building user interfaces (UIs) or user interface components. It was developed and is maintained by Facebook (now Meta Platforms, Inc.) and a community of individual developers and companies. React was first released in 2013 and has since gained widespread popularity in the web development community.

React is primarily used for building single-page applications (SPAs) and dynamic web applications, where the content can change without requiring a full page reload. It allows developers to create interactive, reusable UI components that can be composed together to build complex user interfaces.

Key features and concepts of React include:

1. Component-Based Architecture: React encourages developers to break down the user interface into small, reusable components. Each component manages its own state and can be composed together to create more complex interfaces.

2. Virtual DOM (Document Object Model): React uses a virtual representation of the DOM to improve performance. Instead of directly manipulating the actual DOM elements, React updates a virtual representation of the DOM and then efficiently applies the necessary changes to the actual DOM. This minimizes the number of DOM manipulations and enhances the application's speed and responsiveness.

3. Declarative Syntax: React uses a declarative approach to define how the UI should look based on the application's current state. Developers specify what the UI should look like, and React takes care of updating the actual DOM to match that desired state.

4. Component Lifecycle: React components have a lifecycle that includes methods for initialization, rendering, updating, and unmounting. Developers can hook into these lifecycle events to perform tasks at various stages of a component's existence.

5. State Management: React allows components to have local state, which can change over time. State management can be used to create interactive and dynamic user interfaces.

6. React Router: React Router is a popular library used with React for handling client-side routing in single-page applications. It enables navigation between different views or pages within a web application without full page reloads.

7. React Hooks: Introduced in React 16.8, hooks are functions that allow developers to "hook into" React state and lifecycle features from functional components. This made it easier to manage state and side effects in functional components.

React is often used in conjunction with other libraries and tools, such as Redux or MobX for state management, Axios or Fetch for data fetching, and Babel and Webpack for bundling and transpiling JavaScript code.

React has a large and active community, which has contributed to its ecosystem of third-party libraries, tools, and resources. It is commonly used for building web applications, progressive web apps (PWAs), mobile applications using React Native, and even desktop applications with libraries like Electron.