Helpdesk articles are informative texts for your customers (some people called them FAQs or Knowledge Bases). Canned responses are articles that you can plug right into a ticket response, to save you time. Any article (even unpublished ones) can be a canned response. If you choose to publish an article, it'll appear on your Helpdesk page.
To start creating articles, simply navigate to the Helpdesk "All Articles" view and click the plus button.
Remember: the best canned responses start and end with a personalized message - the canned part is best when it's simply instructions you don't want to have to type over and over.