The <header> and <footer> elements are used at the top or bottom of each web page but can also be used as header and footer of individual <article> and <section> elements. For example, in a blog, a blog post could be an article element and each blog post could have its own header and footer element.
The <nav> element is used for the major navigation elements of a web page.
The <article> element is used to define content that can be considered as standalone information such as a comment or a blog post or an item for sell.
The <aside> element can be placed inside an article element in which case it should have content that relates to the article but is not an essential part of the article. It can also be used outside of an article element, in which case it should contain content that is related to the entire web page.
The <section> element is a grouping of different sections of a web page, so there may be sections of news, products, comments, blogs and so on.