String in rust is roughly like this in C:
// NB: Must be utf-8!
struct string {
size_t sz;
size_t capacity;
unsigned char *buffer;
};
&String in Rust is roughly like `const struct string *`.
str in Rust is just an array of (guaranteed utf-8) unsigned bytes. It does not have a capacity, so it can't be resized. You can't directly construct one (on the stack), because its size is undetermined and Rust doesn't have dynamic-sized stack allocation.
&str, and Box, are pointers to str, along with a size, and are roughly like this C:
// NOTE: Must be utf-8!
struct str_ptr
{
size_t sz;
unsigned char *buffer;
}
The difference between &str and Box is that the latter is an owned pointer to a heap allocation which will be freed when it goes out of scope. &str is unowned and might point anywhere: to a Box on the heap, to a String on the heap, or to read-only static memory.
IMO, it's probably easier to first try to understand the difference between `Vec`, `&[u8]`, and `&Vec`, because they are slightly less "weird" than the string types: they aren't syntactically special like `str` is[1], and they don't have an implicit requirement to be utf8 that is inexpressible in the type system.
[1]: `str` is syntactically special because it is basically a slice, but isn't written in slice notation.