It's a stackless coroutine. AFAIK, the term “green thread” is usually reserved to stackful coroutines, but I guess it could also be used to talk about any kind of coroutines.

It's more efficient (potentially substantially more so). In a typical threaded system you have some leaf functions which take up a substantial amount of stack space but don't block so you don't need to save their state between context switches. In most green threaded applications you still need to allocate this space (times the number of threads). The main advantage of this kind of green threads is you can seperate out the allocation which you need to keep between context switches (which is stored per task)versus the memory you only need while actually executing (which is shared between all tasks). For certain workloads this can be a substantial saving. In principle you can do this in C or C++ by stack switching at the appropriate points but it's a pain to implement, hard to use correctly (the language doesn't help you at all), and I've not seen any actual implementations of this.