If you are wondering, which section element - SECTION or ARTICLE - to use while creating your html markup, just keep in mind this useful and concise definition of both of them and in what way they are different.
Section is to be used, when we talk about something, which is a part of something else. For instance, a chapter in the book. Because, the chapter is part of the book along with other chapters.
Article should be used, when we mean something, which exists on its own. For instance, a tweet, comment, blog post, newspaper ad. Because, comment is not part of the article, which it refers to, the comment lives by itself.