React DOM is the core library within the React ecosystem responsible for rendering React components to the Document Object Model (DOM) in web browsers. It acts as a bridge between React's declarative component model and the imperative browser DOM API. When a React application runs in a browser, React DOM takes the virtual DOM created by React components and translates it into actual DOM elements. It efficiently updates the browser's UI by performing a process called 'reconciliation,' where it compares the new virtual DOM tree with the previous one and applies only the necessary changes to the real DOM, minimizing performance overhead. This library provides methods like `ReactDOM.render()` to mount React components into a DOM node and `ReactDOM.unmountComponentAtNode()` to remove them. It also handles event delegation, attaching event listeners at the document level for better performance and cross-browser consistency.