This is a function invocation:

    foo(bar)

This is a macro invocation:

    foo!(bar)

It's done this way because macros can have much more complex things inside the ()s than functions can. This helps both humans and computers parse such things.

Not OP, I think they mean the exclamation mark (!)

foo(x, y, z) is a function. foo!(x, y, z) is a macro.

    println!("something")

that's a macro

Exclamation mark. e.g.

    println!("hello, world");

or

    todo!();

are macro invocations.

Macros:

#[Foo] struct Bar {}

baz!()

cat!{}

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