ReactJS is an open-source JavaScript library developed by Facebook for building user interfaces, particularly single-page applications where you can build UI components. It’s known for its efficiency, flexibility, and component-based structure.

React allows you to build complex user interfaces by breaking them down into reusable components. It improves performance by using a virtual DOM and makes the development process faster with its declarative syntax. React is also popular and has a vast ecosystem and community.

he Virtual DOM is a lightweight representation of the actual DOM. React uses it to optimize rendering by updating only the parts of the DOM that have changed. It makes React fast, as it avoids unnecessary manipulations of the real DOM.