It depends so much on the verb and the context. For example studying is heavily used for general statements when it's transitive - a student might say "I'm studying English" because that's a subject they're taking at university, even when they've not done any active studying for weeks. Similarly, "I'm sleeping about 8 hours a night" is habitual or general.
The idea that "doing it now" is necessary for the present progressive (and "doing it then" for the past progressive or past perfect progressive), while the present simple is for habitual actions, is one of the most confusion-laden simplifications used when teaching English. It's probably worse than the spelling generalisation "i before e, except after c", which is, I believe, at least true most of the time when you factor in how often each word is used.
"I'm living in Japan at the moment."
"Oh? What are you doing there?"
"I'm Teaching English."
You're not teaching English right at the moment, but it's what you're doing in Japan - your occupation there, or perhaps your apparent reason for being there.
"Gosh, you look tired. Are you okay?"
"I'm not sleeping well at the moment."
Even with the qualification at the moment, this doesn't mean right now, it means in general, recently.
"What do you do with your free time?"
"Oh, I'm writing a book."
Nothing to do with what you're doing right then.
Now, for the first, you could say "I teach English", and it would be fine. In that case, there's no real difference between the two. But you would be causing some confusion by saying "I don't sleep well at the moment", even though everyone would understand the meaning fine, because it's an unusual phrasing (less unusual in some dialects), and without a qualifier like recently or at the moment it would be taken as a much more general, long-term thing. "I'm not sleeping well" would be taken to be short-term without that qualification. Answering a question about hobbies and pass-times with "I write" would be okay, but "I write a book" would be jarring and utterly unnatural.
The difference between the two depends on the context and the verb (and possibly the subject and object).