What is the Tokio runtime and how is it used in Rust?

Tokio is the most popular async runtime for Rust, providing an executor (polls futures to completion), async I/O (TCP, UDP, files via tokio::net, tokio::fs), timers (tokio::time::sleep()), synchronization primitives (tokio::sync::Mutex, channels), and a thread pool for CPU-bound tasks (tokio::task::spawn_blocking()). Annotate your main function with #[tokio::main] to run it as an async function inside a Tokio runtime. Spawn concurrent tasks with tokio::spawn(async { ... }) — tasks are lightweight (similar to goroutines). The Tokio ecosystem (axum, reqwest, sqlx) all build on Tokio, making it the de-facto standard for async web services and network programs in Rust.