Ad and Content Marketing both have to do with Content. There are a few differences in the nature of Content between the two but the key difference is not about Content per se but about its Placement. Ad is Content placed in Paid Media whereas Content Marketing is Content placed in Owned Media or Earned Media. Ad industry has been using the term terms “Copy” for textual content and “Creative” for graphical content. But that doesn’t take away from the fact that Ad is still Content.
Advertorial / Native Ad is a little tricky. It is placed in Paid Media. But its nature of content is not at-your-face like Ad but educational leading to brand specific virtues. It’s difficult to say with certainty whether Native Ad is Ad or Content Marketing.