Semantic markup, also known as semantic html, is defined by Wikipedia as "the use of HTML markup to reinforce the semantics, or meaning, of the information in web pages and web applications rather than merely to define its presentation or look." It has been a part of HTML since it was first created. Semantic tags are used to separate the contents of a web page by type and helps to organize the source file as well as to create a level of abstraction from the presentation of the content.
Section 508-Accessibility are the standards by which federal agencies must adhere to when creating a website. These standards are what screen readers and other assistive technology rely upon to understand the content of a web page. A website is considered to be complaint to section 508-Accessibility if it meets the requirements found in parts a-p outlined in Section 1194.22. While it is not required for non federal websites to comply to these standards as well, it is considered good practice to do so.
Search engine optimization or SEO, is defined by Wikipedia as "..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". Essentially what this means is, tailoring your website so that a search engine will put your site at the top of the results.
Today popular search engines such as Google and Bing, scans and indexes the semantic markup of published web pages. When a query is submitted to a search engine, it searches the index for keywords and produces a list of sites that best relates to the question. In order to appear near the top of the list, a web site needs to be Section 508 compliant as well as properly optimized. Proper optimization can include using meaningful header tags and emphasizing common keywords throughout the markup.