This article is about sample articles. If it were about anything else it would be called something else, but it would be correct to call it "Something Else" only if it were actually about something else. Remember that if you are writing an article: the title of the article is what you write about, and anything else belongs in an article about something else, otherwise it could get confusing for the person who wants to read about something else, and who looks for an article called "Something Else" but ends up reading about anything else.
This is not the introduction to this article. The introduction to this article is the paragraph which you read at the beginning of this article. If this article would have been about introductions (and in that case the article would be titled "Introduction"), then this paragraph would have been put at the beginning and it wouldn't have a sub-heading, as this paragraph has, because introductions normally don't use sub-headings called "Introduction" because that's what they are, and we don't have to be told.
This is not a sub-heading. The sub-heading is the phrase above the line before this paragraph which says "Sub-headings." The sub-heading tells you what the next few lines are about, as in this case, the "next few lines" are about the line that immediately precedes them; that's what sub-headings do. You can identify a sub-heading by what it looks like: it is made up with letters that are a bit bigger than the letters you read following it. Although you don't see them here, the sub-headings have two equals signs (=) which you type in at the start of the line and after the sub-heading when you are creating or editing an article and these equals signs magically disappear when the article is reproduced.
Other sub-headings are not called "Sub-headings" just as the first sub-heading shouldn't be called "Sub-headings" either (unless, of course, you intend to write about "sub-headings"). The sub-heading you use tells you what to write about in the next few lines and if you use more than one of them, then you have to write about more than one thing. When you run out of things to write about, then you don't need any more sub-headings except, possibly, one called "See Also."
See Also is a special kind of sub-heading that you use when you are too lazy to write anything else about what you're writing about, and you have the sadistic desire to send your readers off on a wild-goose chase in the hopes that maybe you will direct them to the information which you didn't give them but which they still want. The see also sub-heading is also special in that it is called "See Also," however, you do not write about "see also" in the next few lines, but you write other things. See Also usually has links to featured articles so that the writer can give the illusion of his article being related to a quality article in some way.
External Links are another special kind of sub-heading that you don't have to use if you are feeling particularly secretive and don't want to share anything more than you have to with the person who reads your article. Using external links is also a sign of laziness on your part because if you were much more industrious you would go to the external link yourself, summarize the pertinent information, put it into clear, understandable language and include it in your article.