What Is the Difference Between Concurrency and Parallelism in Rust?

Concurrency is managing multiple tasks at once by switching between them, while parallelism is executing multiple tasks simultaneously on different CPU cores. In Rust, concurrency often uses async and await to pause and resume tasks without blocking, whereas parallelism typically uses threads to run code at the same time.

// Concurrency: One task pauses, another runs (async/await)
async fn download_file() { /* ... */ }

// Parallelism: Multiple tasks run simultaneously (threads)
std::thread::spawn(|| { /* ... */ });

Concurrency is like a single chef chopping vegetables, then pausing to stir a pot, then chopping again; they handle multiple dishes but only do one thing at a time. Parallelism is like having two chefs working in the kitchen at the same time, each chopping their own vegetables simultaneously. You use concurrency to keep your app responsive while waiting for data, and parallelism to speed up heavy calculations by using all your computer's power.