Call hooks at the top level only, never in loops or conditions. Only call hooks from React functions. These rules exist because React tracks hooks by call order, and breaking the order causes subtle bugs.

It runs side effects after render. Dependency array: empty runs on mount, with values runs on change, missing runs every render. Return a cleanup function for subscriptions and timers to prevent leaks and stale updates.

React's in-memory description of the UI. It compares the new description to the previous one and updates the real DOM only where they differ, a process called reconciliation, which makes updates efficient.