Start with the idea
Rust requires every parameter type to be written, which lets the body and every call be checked independently. Arguments are the concrete values passed at a call site; parameters are the named inputs in the definition. Passing an owned type may move it, while a reference lets the function borrow it — a distinction later lessons develop in depth.
In fn area(width: u32, height: u32), width and height are parameters: names the function uses inside its body. In area(4, 3), 4 and 3 are arguments: values supplied by one call. Rust checks that each argument can be used as the corresponding declared type. A literal often gains its type from this context, so you do not have to bind 4 to a separate u32 variable before calling area.
Order matters even when types match. area(4, 3) and area(3, 4) happen to return the same number, but a function such as subtract(left, right) would not.
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