ReactJS, commonly referred to as React, is an open-source JavaScript library for building user interfaces (UIs). It was developed by Facebook and is widely used for creating dynamic and interactive web applications.

React follows a component-based architecture, where the UI is broken down into reusable and self-contained components. Each component manages its own state and renders a part of the user interface based on that state. This approach promotes reusability, modularity, and makes it easier to manage complex UIs.

Key features and concepts of React include:

1. Virtual DOM: React uses a virtual representation of the DOM (Document Object Model) in memory. When the application state changes, React efficiently calculates the minimal number of changes required to update the actual DOM, resulting in better performance.

2. JSX: JSX is a syntax extension for JavaScript that allows developers to write HTML-like code within JavaScript. It enables the combination of HTML markup and JavaScript logic, making it easier to describe the structure and behavior of UI components.

3. Unidirectional Data Flow: React follows a unidirectional data flow, also known as one-way binding. Data flows from parent components to child components via properties (props) and is typically not mutated directly by child components. This approach simplifies debugging and makes the application more predictable.

4. Component Lifecycle: React components have a lifecycle consisting of various methods that are executed at different stages. These methods allow developers to perform initialization, update, and cleanup operations at specific points in a component’s life.

5. State Management: React components can have state, which represents data that can change over time. State allows components to be dynamic and interactive. When the state changes, React automatically re-renders the affected parts of the UI.

6. React Router: React Router is a popular routing library for React applications. It enables developers to define and manage different routes within a single-page application, allowing for navigation and rendering of specific components based on the current URL.

React has a large and active community, which has contributed to the creation of numerous third-party libraries and tools that enhance React development, such as Redux for state management, React Native for mobile app development, and Next.js for server-side rendering.

Overall, React has gained significant popularity due to its declarative and component-based approach, efficient rendering with the virtual DOM, and the ability to build reusable UI components.