Async vs parallel
Async: Do this by yourself somewhere else and notify me when you complete(callback). By the time i can continue to do my thing.
Parallel: Hire as many guys (threads) as you wish and split the job to them to complete quicker and let me know (callback) when you complete. By the time i might continue to do my other stuff.
Taken from here.
This is an ongoing article, that i will update with new info when i find good explanations about the subject.