Explain what Virtual DOM is in React.

Virtual DOM in React is an abstract representation of the real DOM in memory. Instead of directly modifying the real DOM on each update, React first updates the Virtual DOM, then compares it with the previous version (a process called "reconciliation").

After comparison, React calculates the minimal set of changes needed to update the real DOM and applies only those. This improves performance because operations on the real DOM are costly and slow.

It works roughly as follows:

1. The component's state changes.
2. React creates a new Virtual DOM.
3. It compares the new Virtual DOM with the previous one.
4. It finds the difference (diff).
5. It updates the real DOM only where necessary.

Thus, the Virtual DOM is an optimization layer that allows efficient interface updates.