Explain Virtual DOM in React.

Virtual DOM is an abstraction of the real DOM used by React to optimize interface updates. Instead of directly modifying the browser's DOM, React first updates a virtual representation of the element tree in memory, compares it with the previous version (a process called "reconciliation"), and calculates the minimal set of changes needed to update the real DOM. This improves performance and reduces the number of costly DOM operations.

Example:

const element = <div>Hello, world!</div>;
// React creates a virtual tree of elements
// When the state changes, React updates the virtual DOM,
// compares it with the previous version, and applies minimal changes to the real DOM.