Tell me about Virtual DOM and React Fiber: what are they, and how are they related?

Virtual DOM is an abstraction of the real DOM that React uses to optimize interface updates. Instead of directly modifying the DOM, React first creates a virtual tree of elements in memory, compares it with the previous version (a process called "diffing"), and calculates the minimal set of changes needed to be applied to the real DOM. This significantly improves performance because operations on the real DOM are costly.

React Fiber is a new architecture of React's internal engine that handles scheduling and executing updates to the Virtual DOM. Fiber breaks down work into small units (work units), allowing React to pause and resume rendering, as well as prioritize important updates (such as user events) over less critical ones (like background updates).

Thus, Virtual DOM is a concept of representing UI in memory, and React Fiber is the mechanism that manages updating this Virtual DOM and efficiently applying changes to the real DOM.