What is the difference between libc++ and libc++abi library in LLVM?

Question

I saw the two projects are quite related, but what are the differences between them? The official webpage doesn't tell much about it.

I know that ABI (Application Binary Interface) is used to provide low-level binary interface among different platforms. So is libc++abi used to provide different implementations for different platforms, and general interface for libc++?

Would be better go give some specific example, e.g. what are included in libc++abi and what in libc++.

Thanks.