Rust: What is the difference between Cow<str> and String?

1use std::borrow::Cow;
2
3fn get_greeting(name: &str) -> Cow<str> {
4    if name.is_empty() {
5        Cow::Borrowed("Hello, stranger!")
6    } else {
7        Cow::Owned(format!("Hello, {}!", name))
8    }
9}
10
11let greeting = get_greeting("");
12let greeting = get_greeting("Alice");