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.
Semantic markup has maintained its useflness by being used widely by search engines to properly categorize, summarize, and display information during a web search.
Section 508 is a portion of the amended Rehabilitation Act of 1973. Standards defining web accessability to those who are disabled was added, to establish rules on the internet that allow them to use the web. In short it requires federal agencies to adopt policies to make electronic information accessable to those with disabilities at a comparable level to those without. Others are not required to use these standards, but it is extremely bad online etiquette not to do so.
SEO - Search Engine Optimization
Wikipedia defines Search Engine Optimazation as:
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.
To become more visible on search engines a number of tactics are used by websites. Some of which could be considered unethical, such as falsifying network traffic or information provided by a website. Search engines use complex algorithms to process the large amount of information on the web, and use these algorithms to combat these unethical practices as well. Search engines also use these algorithms to "prop up" websites displaying good web etiquette.
How are they related?
Semantic markup is used in Search Engine Optimization to properly identify important information contained within a web page. This can be taken advantage of as stated previously, and has been mitigated by modern search engines. Search engines optimize their results to boost websites that display Section 508 accessability standards, and in this way help propogate and universalize those same standards. Finally proper use of Semantic Markup is an integral part to Section 508 Accessability, because it involves identifying important aspects of the information displayed. This is invaluable to accessability software to help the disabled access web content.