Callback hell is what you get when you take a language designed for doing at most client-side input validation, and try to write serious applications in it.
As an analogy, consider what would happen if Outlook was written in VBA and you opened OUTLOOK.DOC in Word to read your email. Well that's GMail.
As an analogy, consider what would happen if Outlook was written in VBA and you opened OUTLOOK.DOC in Word to read your email. Well that's GMail.