Unsized Types

There is a new edition of the book and this is an old link.

Sometimes referred to as ‘DSTs’ or ‘unsized types’, these types let us talk about types whose size we can only know at runtime. The Sized trait is automatically implemented for everything the compiler knows the size of at compile time. A trait bound on ?Sized is the opposite of a trait bound on Sized; that is, we would read this as “T may or may not be Sized”.

fn generic<T: ?Sized>(t: &T) {
    // ...snip...
}

You can find the latest version of this information here.