Let's talk about packages and crates. Here's a summary:

• A "crate" is a binary or library.
• The "crate root" is a source file that is used to build a crate.
• A "package" has a Cargo.toml that describes how to build one or more crates. At most one crate can be a library.

So when we type cargo new, we're creating a package:

> cargo new foo
> ls foo
src
.gitignore
Cargo.toml
> ls foo/src
main.rs

There's a Cargo.toml, that checks out. And while there's no description of main.rs inside of it, by convention, if you have a src/main.rs in the same directory as a package's Cargo.toml, Cargo understands that to be a binary crate with the same name as the package. Likewise, with src/lib.rs, Cargo knows that's a library crate with the same name as the package.

What exactly is a crate in this case? Well, we call the main.rs or lib.rs file the "crate root", that is, the source file that corresponds to the crate. This file is passed by Cargo to rustc in order to actually build the library or binary.

A package can have:

• Zero or one library crates
• As many binary crates as it would like
• There must be at least one crate

If we have both src/main.rs and src/lib.rs, then our package has two crates: a library and a binary, both with the same name. If we only had one of the two, we'd have either a single library or binary. But what about more than one binary? We'll talk about that more in Chapter 14, "More about Cargo and Crates.io."

That's all we'll say about packages until then. And to learn more about crates, we need to understand "modules." Read on to find out more!