New HTML5 structural tags
<header> -- Use for page or section headers. There can be mutliple headers, but you cannot place headers within headers. It is appropriate to place H1-H6 tags, hgroup tags, and nav tags within header tags.
<hgroup> -- Use for grouping related heading elements, such as a an H1 for a title and an H2 for a tag-line. Only the H1 in this case would be represented in the content outline.
<footer> -- Use for page or section footers. There can be mutliple footers, but you cannot place footers within headers.
<nav> -- Use for defining a section intended for navigation. It should contain navigational links.
<main> -- Use to represent the main content of the <body> of a document or application. This content should be unique to the document and not repeated elsewhere. The main tag must not be nested in a section or article tag (but would more likely be the parent of such tags). There must not be more than one <main> tag within a document.
<section> -- Used for grouping together thematically related content and should contain (and be able to be described by) a heading element (either h1-h6 or hgroup or header).
<article> -- Used for "self contained" content that may be potentially syndicated such as blog posts, news stories, comments, reviews, forum posts, or widgets.
<aside> -- Use for sidebar content, tangentially related to page or section content. Note that "aside" doesn't necessary connote its position on the left or right side (although it may indeed be positioned in such places), but rather suggests that its content is tangentially related (but not essential) to the primary content of the section and/or page in which the aside resides.