An article is an independent, stand-alone piece of discrete content, sort of like a blogpost or a news item, a self-contained composition. It should be able to stand on its own as if it was independent of its surroundings and that can be independently distributable or reusable.
Section, on the other hand, is either a way of sectioning a page into different areas, or sectioning an article into sections. basically it is a thematic grouping of content, usually with a heading.
Section can be used to group related articles together. It helps to think of a web page as a newspaper that has different sections: sports, real estate, etc. with a number of articles in each section.
Each section should have a heading to identify that secion's content. If you don't use a heading in your section element, you most likely should not use the section element. In that case a div might be more appropriate.