What is Semantic Markup?
The word semantic means of or relating to meaning. "For language this is the words we use to communicate. For HTML it is the tags we use to mark a document up. The word markup means the way a web document is created, or the way you write a document using the language available to you (10)." As a result "semantic markup" in terms of html is the systematic way of organizing a web page using tags that helps the browser "read" the html in order to create the web page that you wish to see.
What is "SEO"?
SEO stands for search engine optimization. It refers to how semantic markup is used to make sure that a search engine can find your web page. It also makes sure that search engines can understand the contents of your web page and then the search engine can rank your web page among similar pages based on key words. As a result using semantic markup on your web pages is what makes a search engine more likely to show your website as a search result.
What is Accessibility?
Also known as Section 508: Accessibility, is enforced by the EPA and states "Section 508 of the Rehabilitation Act (29 U.S.C. § 794d), as amended by the Workforce Investment Act of 1998 (P.L. 105-220) requires federal agencies to develop, procure, maintain and use electronic and information technology (EIT) that is accessible to people with disabilities - regardless of whether or not they work for the federal government (11)." Essentially what this means is that a website must be equally accessible to people including people with physical, sensory, or cognitive disabilities.
How are all these topics related?
If you are making a website and want your site to be viewed as much as possible, you have to take all of these things into account. Most of the traffic a site generates is through search engines using "SEO". Semantic markup becomes important since "the only way search engines can understand which elements on the page are more important than others is through the markup. "For example, if the topic of your page is within a 'Paragraph' tag it is likely to get lost in other content and not be returned in search engine results. But if you put it within a 'Heading 1' tag the search engine know to give it more importance." So search engine optimization is one very good reason to use semantic markup (10)." As well, the semantic markup takes into account the rule of Section 508: Accessibility because the semantic markup is already designed to to write web pages that are easily readable and accessible by anybody including those with disabilities that wishes to use them. An example of this might be the use of an 'alt' description for a picture.