Semantics, really.

Guest blogging is when an article you’ve written appears on someone else’s blog. You are referred to as the ‘guest blogger’ in this situation.

Guest posting is another way of saying guest blogging. A ‘post’ being the article that appears on the blog.

Article marketing is what you do when you’re trying to get your content (articles) on a website other than your own.

So guest blogging/posting is simply a form of article...