Semantic Markup Explained
Semantic HTML is the use of HTML markup to reinforce the semantics, or meaning, of the information in webpages and web applications rather than merely to define its presentation or look. Semantic HTML is processed by traditional web browsers as well as by many other user agents. CSS is used to suggest its presentation to human users.
Section 508 Accessibility
The Board’s Section 508 Standards apply to electronic and information technology procured by the federal government, including computer hardware and software, websites, phone systems, and copiers. They were issued under section 508 of the Rehabilitation Act which requires access for both members of the public and federal employees to such technologies when developed, procured, maintained, or used by federal agencies.
Search Engine Optimization
Search engine optimization (SEO) is the process of affecting the visibility of a website or a web page in a web search engine's unpaid results—often referred to as "natural", "organic", or "earned" results. In general, the earlier (or higher ranked on the search results page), and more frequently a site appears in the search results list, the more visitors it will receive from the search engine's users.
How It All Relates
These three concepts are used in tandem in order to rank and organize web pages. A page with a well written semantic markup will appear earlier in search engines through the use of Search Engine Optimization. In addition to this, if the semantic markup is written properly, it will be able to classify as Section 508 accessible. This will make your webpage accessible to people with disabilities due to the ability for the page to properly read your semantic markup.