Java Threads are essential for running tasks in parallel — they let your program do more than one thing at a time. The Thread class is your starting point, but Runnable is often used for cleaner design. Threads share memory, which is powerful but risky — race conditions and deadlocks can happen fast. Always synchronize access to shared data. Use ExecutorService instead of manually creating threads — it handles pooling and reusability. And remember, just because a thread exists doesn’t mean it’s running — the scheduler decides when it gets CPU time. Mastering threads means mastering real-world performance.