[steveklabnik]

I don't know why you're downvoted.

> But what are they?

The language's semantics are such that by default, you get memory safe code. This is checked at compile time. While many languages are memory safe, they often require a significant amount of runtime checking, with things like a garbage collector. Rust moves the vast majority of these kinds of checks to compile time, and so has the performance profile of C or C++, while still retaining memory safety.

> How does it compare with modern C++?

One way to look at Rust is "modern C++, but enforced, and by default." But that ignores some significant differences. For example, Rust's Box<T> and std::uniq_ptr are similar, but the latter can still be null, whereas Rust's can't. C++ cannot be checked statically for memory safety, even if modern C++ helps improve things, it doesn't go as far as Rust does.