The HTML5 specification includes a series of new semantic elements that is used to
give some meaning to the various sections or parts of a Web page, such as a
header, footer, navigation, and so on. In previous versions of HTML (HTML4.01), you would
typically use <div> elements to create these parts, using ID or class attributes to
differentiate them from each other. The problem with this is that this has no semantic
meaning, as there are no strict rules defined that specify what class names or IDs
are to be used, making it extremely difficult for software to determine what the
particular area is doing. HTML5 should help alleviate these issues, making it easier
for Web browsers to parse the semantic structure of a document.
It is worth pointing out that continuing to use <div> elements in HTML5 is perfectly
valid, but in order to future-proof your work, it is recommended that you use
semantic elements where relevant.

Try avoid using these new elements for purposes other than their intended. For
example, the <nav> element should not be used for just any group of links; it is
intended to surround the main navigation block on the page.
The main semantic elements that HTML5 introduces are:

<header>

This element is used to define a header for some part of a Web page, be it the
entire page, an <article> element, or a <section> element.

<footer>

Like the <header> element, this new element defines a footer for some part of
a page. A footer does not have to be included at the end of a page, article, or
section, but it typically does.

<nav>

This is a container for the primary navigation links on a Web page. This
element is not intended for use with all groups of links and should be used for
major navigation blocks only. If you have a <footer> element that contains
navigation links, you do not need to wrap these links in a <nav> element, since
the <footer> element will suffice on its own.

<article>

The <article> element is used to define an independent item on the page that
can be distributed on its own, such as a news item, blog post, or comment.
Such items are typically syndicated using RSS feeds.

<section>

This element represents a section of a document or application, such as a
chapter or a section of an article or tutorial. For example, the section you are
reading now could be surrounded by a <section> element in HTML5. <section>
elements typically have a header, although it is not strictly required. The header
for the section you are reading now would contain the text "Semantic
elements," for example.

<aside>

This new element can be used to mark up a sidebar or some other content that
is considered somewhat separate to the content around it. An example of this
might be advertising blocks.

<hgroup>

In some cases, a page, article, or section may require more than one heading,
such as where you have a title and a subtitle. This tutorial, for example, has the
title "Create modern Web sites using HTML5 and CSS3" and the subtitle
"Implementing the canvas and video elements in HTML5." You could wrap
these in an <hgroup> element, using an <h1> element for the main title
Create modern Web sites using HTML5 <h2> element for the subtitle.
The sample Web site at the end of this tutorial includes several of these new
semantic elements, and I will explain their syntax and use in more detail at that
point.