I have an individual article page; to return back to the article listing page, I'd like to use a back button. Where should a back button typically be positioned? Is it in the top left hand corner?
Your first guess is in general a good guess. Yet there are some "details" you need think of.
First, it has to follow the overall structure of your website (e.g. when you're in your list of articles, where is the "back to home" button?). Generally, once a user went through 3-4 pages in your website, he will get a mental model of how it is structured and how it should work. Of course, the best (read "for the user") would be to create a website where the user is not creating a new mental model because he already has one (e.g. create a website that looks like amazon).
Then, it will also depend on who your users are? And on what kind of device they are using (computer vs. smartphone). On a computer, as Kayo suggested, a good approach is to have a top and a bottom button. On smartphones, it is more appropriate to have a navigation bar that is always displayed (either at the top or at the bottom, again depending on your entire project).
If you don't have a particular website with an already defined structure, I would go for the top-left (and bottom) side button since it will follow the mental model people have of the browser itself which always has the back and forth button on the left of the navigation bar.
Finally, always try to think not only about one particular button, but to stand back and think about all needed navigation buttons. Imagine for example adding the
I would put a back button at the top left and bottom left of the article content.
This way, if the user reads just a little and decided they do not want to continue reading they can immediately return.
Alternatively, if they read to the bottom of a very long article, they do not have to scroll back to the top in order to return.
I would also consider not labelling the button 'Back'. But perhaps 'Back to articles' or whatever would be appropriate to the content. As 'back' is relative to where the user came from, someone might of followed a link to an individual article from outside of your website.
Also if you do implement a back button I would suggest to give the users more information, so rather than just saying "Back" it could say "Back to [insert subcategory page here]". Just so that it better orients your users.