"Should" and "Ought to"

I hadn't known the difference between "should" and "ought to". Many English grammar books say about these, but I had always thought those were the same and I wondered what is different.

Now, I have been studying English grammar using "Grammar in Use Intermediate", which is one of the "in Use series" published by Cambridge University Press. Those are very famous and used in the world. Of special note is (at least for me) that it was written by a native speaker in an English-speaking country. Many English studying books were written by Japanese in Japan. Sometimes native speakers point out mistakes or unnatural expressions in example sentences in those books.

In the Grammar in Use Intermediate, it says that you can use "ought to" instead of "should". Oh--! So I couldn't understand the difference. Oh, those are the same! Thank you, the book.