Semantic HTML is a way of using HTML coding to create or enhance the structure of a page. In other words, it's a way of using HTML markup - classes, divs, tags and so forth - to complement the actual words or resources on a page. This helps 'bots' and visitors using screen readers to understand the structure and context of the information on the page, along with its importance, relevance, and how it is related to other resources.
It is important to have an understanding of Semantic HTML if you are developing websites or writing content for them, as you will need to use the structural markup regularly.
An example of misuse of Semantic Markup can be found when an article has been written using normal text, but at some point in the text the writer wants to emphasise a particular phrase. They like the styling of the H1 tag, so they apply H1 to this phrase to 'make it look pretty'. Unfortunately, this is confusing to a search engine 'bot' and to users of screen readers, because they are told that this is the main heading of the page - rather than emphasised or important text.
Semantic HTML markup should only ever be used to add structure to a page - not to change the way it looks (this is done using Cascading Style Sheets (CSS) or in-line styling)
For example, lets say we have an article:
<h1>Using headings</h1> This is an article about the importance of headings <h2>Why use headings?</h2> It is important to use headings so that search engine bots can tell what is an <strong>important</strong> part of your article <h3>Types of headings</h3> You can use set types of headings, but they should be ordered, and structured, within your page. H1 should be your page title, with H2 being used for sub-headings of the page. Any headings within your sub-headings should cascade using H3, H4, and H5 as appropriate. <h2>Is it hard to implement headings?</h2> It is really easy to implement headings, you just use the appropriate HTML code <h3>Using headings on dynamic pages</h3> On dynamic pages, simply wrap your main heading within a H1 (for example, the title of a category listing page would be H1) then wrap all subsequent headings in H2.
Here, a search engine 'bot' could clearly see the structure - h1, h2, h3 - but if we were to simply make these titles bold, underlined and larger font, it would be much more difficult to identify the structure. It is also possible to identify that the word 'important' is an emphasised word, something that is important within the page.
Semantic HTML is also
In the example provided below of this page in Google search results, you can see how the heading tags are being used by Google to identify smaller links within the main page which may be of interest to the person searching for a term (displayed in small blue hyperlinked text beneath the description) - another reason to ensure you structure your content well!
Microdata is a more advanced form of Semantic HTML Markup which allows you to give even more contextual information about the content and structure of your website - for more information start with this page on Microdata.