The <article> tag defines content that can be used independently, that is, it contains content that has value on its own. For example, in a news website, it will contain a number of news, but each news is an article on its own. This example is similar to a blog where the main page may contain a number of blogs whereas each blog is an independent article. Each article tag should contain a heading element to identify itself.
Note that the <article> can be nested meaning that the inner <article> is related to the outer <article> tag. Also note that, the contact information of an article can be defined by using a <address> tag, though it does not apply to the nested <article> tags.