Fibers and coroutines are different uses for the same thing. If you schedule your coroutines based on the kernel scheduler, they're fibers. If you jump between fibers explicitly, they're coroutines.
I think it's mainly a fiber library, actually. The TCP and channel functions yield and resume automatically based on when the underlying operation blocks.
They have yield(), but I don't see the opposite, the ability to explicitly resume a stopped context.
Edit: for a "pure" coroutine library, I'd recommend libco from BSNES. It only has 4 functions: active, create, switch and delete. I wrote a fiber library using libco plus libuv, which was fairly easy.
