React is an open-source JavaScript library developed and maintained by Facebook (and a community of developers) for building user interfaces, particularly for web applications. It's widely used to create dynamic, interactive, and reusable UI components.

The primary goal of React is to provide a flexible and efficient way to create user interfaces that update and render efficiently when the underlying data changes. It uses a concept called the "Virtual DOM" to optimize the process of updating the actual browser DOM, making UI updates more performant.

Key features and concepts of React include:

1. Components: React applications are built using components, which are modular, reusable building blocks for UI elements. Components can be composed together to create complex user interfaces.

2. Virtual DOM: React uses a Virtual DOM to represent the structure of the actual browser DOM. When data changes, React calculates the difference between the previous and new Virtual DOM states (known as the "diffing" process) and updates the actual DOM efficiently.

3. One-Way Data Binding: React enforces a unidirectional data flow. Data flows from parent components to child components via props, and child components can't directly modify the data passed to them.

4. JSX: JSX is a syntax extension that allows you to write HTML-like code within your JavaScript code. It makes it easier to describe the structure of your UI components.

5. State: React components can have internal state, which allows them to manage and respond to changes in their own data. Stateful components re-render when their state changes.

6. Props: Props (short for properties) are data passed from parent to child components. They are immutable and allow you to configure and customize components dynamically.

7. Hooks: Introduced in React 16.8, hooks are functions that allow you to "hook into" React state and lifecycle features from functional components. They provide a more concise way to manage state and side effects.

8. Lifecycle Methods: In class components, lifecycle methods allow you to perform actions at specific points in a component's lifecycle, like mounting, updating, and unmounting.

React has gained significant popularity in the web development community due to its simplicity, reusability, and performance optimizations. It's often used in combination with other libraries and frameworks (like Redux for state management) to build comprehensive web applications.