Web Workers allow you to run JavaScript in a separate thread, parallel to the main thread. This means heavy computations won't freeze your user interface.
To create a worker, pass a script URL to the Worker constructor. Communication happens through postMessage() and onmessage. Workers cannot access the DOM directly.
MessageChannel creates private two-way communication between the main thread and a worker. Transferable objects like ArrayBuffer can be transferred without copying, giving near-zero cost data sharing.
SharedArrayBuffer enables true shared memory between threads, with Atomics for safe concurrent access.
Practical use cases: image processing, CSV/JSON parsing of large files, search indexing, cryptographic operations, and any CPU-intensive task. Libraries like Comlink make worker usage as simple as calling an async function.