There is a new edition of the book and this is an old link.
Rust has a module system that enables the reuse of code in an organized fashion. A module is a namespace that contains definitions of functions or types, and you can choose whether those definitions are visible outside their module (public) or not (private).
A crate is a project that other people can pull into their projects as a dependency.
Here are the relevant sections in the new and old books: