Thanks for the links, zcorpan! The examples they contain made everything so much clearer.
It was the w3Schools comment about external content that really threw me, but I know better now!
Just one thing I am not sure about:
A page has a list of blog posts. The list gives a lengthy preview of each post, including 2-3 paragraphs, a heading and a photo. Which mark up would be more appropriate:
1) Placing the entire list in <article> and then each preview in a <section>, as the list is essentially one big article with lots of different sections.
2) Placing the entire list in a <div> and then each preview in an <article> (because the list is made up of lots of articles, even if they are just previews).
3) Placing the entire list in <Ul> and then each preview in <li>. This seems the most logical (as it is a list), but the mark up would get messy with trying to put several paragraphs, headings and image tags inside an <li> (and I don't think it is good practise to put all those tags in a <li>).
For a page full of blog posts you would use an <article> for each post (and then wrap the whole lot in a single DIV), but the above example is different as it is a list of previews.