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useEffect is React's mechanism for handling side effects like API calls, event listeners, and DOM manipulation after rendering. The hook supports three behaviors: running on every render (no dependencies), once on mount (empty array), or when specific values change (dependency array). Proper cleanup functions prevent memory leaks by removing listeners and clearing timers. Common pitfalls include infinite loops from updating state based on its own dependency, which can be avoided using functional updates. Real-world applications include data fetching, document title updates, timers, event listeners, and localStorage synchronization.