I've been thinking about how to answer this one, because most of the explanations I've seen have been vague and woolly, especially when it comes to connection with the written word.
One of the other knowledgeable members will correct me if I am wrong in the following, but this is how I tend to think of "contents", versus "content.
To me, in their simplest forms:
1. Contents = Physical items, which are contained within something (e.g. Solid objects like keys in a pocket, money in a jar, photos in a drawer etc; but also the written word as calligraphy stripped of it's meaning, the pages of a book, the articles in a magazine stripped of their meaning).
"Please empty the contents of your pockets" (physical objects)
"The man emptied his suitcase of its contents" (physical objects)
"The file of coursework fell to the floor, and its contents spread like confetti" (The pages here are physical objects)
2. Content = Non-Physical things, (e.g. Ideas, theories, principles, beliefs, arguments or discussion, one could say the "meaning" contained within the words. The ideas and principles that enable learning from a course. The course may be in written form but the content is the ideas "contained" within the meanings of those words).
"The pamphlet contained political content." (non-physical. The words of the pamphlet "contained" a political message)
"The content of the book was heavy-going." (non-physical. The meanings contained in the words required the reader to do a lot of thinking)
"The content of the course was excellent". (non-physical. The ideas, theories, principles on which the course was based enabled the learner to develop new skills).
The one most people have difficulty with is trying to classify writing in a book:
To me the physical pages, writing, paragraphs and words, are the contents of the book.
The content of the book is the ideas, arguments, theories, or teachings, contained within the words on the page.
That's how I see the distinction in general, but others may be able to explain it more clearly, or say if I am wrong.
Student or Learner