String (the “owned” sort of string type) is a wrapper for a heap-allocated buffer of unicode bytes. str (the “slice” sort of string type) is a buffer of unicode bytes that may be on the stack, on the heap, or in the program memory itself.
"unicode bytes" aren't a thing; bytes implies encoding (and subscripting yielding <=0xff), otherwise it's "codepoints" (and subscripting yielding an int somewhere on the unicode planes).
Further down:
String and str are guaranteed to be valid UTF-8 encoded Unicode strings. If you’re wondering why UTF-8 is the standard encoding for Rust strings, check out the Rust FAQ’s answer to that question.
That's spot on. Please add this to the first part, too; "... buffer of UTF-8 encoded unicode bytes". or even just "encoded unicode string." It will be clear what is (and is not) meant.
Otherwise nice article! Even understandable for someone with no Rust experience.