What Is the Difference Between use, mod, and extern crate?

use brings items into scope, mod declares a module, and extern crate (deprecated in modern Rust) declares a dependency from outside the current crate.

mod my_module { pub fn hello() {} }
use my_module::hello;
// extern crate is no longer needed for most dependencies in Rust 2018+

In modern Rust (2018 edition and later), you typically only use use to import items and mod to define modules; extern crate is automatically handled by the compiler for dependencies listed in Cargo.toml.

Think of mod as creating a new folder for your code, use as opening that folder to grab a specific file, and extern crate as an old way to say "I need a library from outside my project." Today, you mostly just use mod and use because Rust automatically handles external libraries.