Bob Nystrom’s two-part blog post about iteration in programming languages includes perhaps the clearest explanation of coroutines I have read so far. It begins with an exploration of how iteration is implemented in mainstream programming languages, and goes on to talk about internal and external iterators, the yield statement in C# and Python, the callstack, coroutines in Ruby (or fibers, as Ruby likes to call them) and why iteration is another way of thinking about concurrency.

Read Iteration Inside and Out and Iteration Inside and Out, Part 2.