Wherever, Whenever, Forever
According to Dictionary.com, Semantics is basically referred to as the correct interpretation of the meaning of a word or sentence. To use the word semantically it is to use it in a way that it's properly aligned with the meaning of the word. So bascially when we misuse a word we are NOT using it semantically. Many HTML tags have a semantic meaning. For example when an h1 heading it interprets that tag to mean that the contents of the h1 element constitute the most important heading of the section that contains the element. The semantic meaning of an h1 tag is that it's used to identify the most important header of a specific web page or section.
Good CSS can make a bad markup invisible to the average website visitor. Even with that no amount of styling will make a bad markup more meaningful to a computerized visitor such as a search engine web crawler, browser translation tools, or assitive technologies such as screen readers. According to Bruce Lawson,the use of semantic HTML elements "enhances accessibility, searchability, internationalization, and interoperability."
Before going into what Section 508-Accessibility lets get some brief history on what it really means. Section 508 is an amendment to the United States Workforce Rehabilitation Act of 1973 which is a federal law mandating that all electornic and information technology developed, procured, maintained, or used by the federal government to be accessible to people with disabilities. Technology is basically meant to be accessible if it can be used as effetivly by people with disabilites as by those without.
What is SEO? SEO stands for "search engine optimization." It is the process of getting traffic from the "free," "oragnic," "editorial" or "natural" search results on search engines. All major search engines such as Google, Bing, and Yahoo have primary search results, where web pages and other content such as videos or local listings are shown and ranked based on what the search engine considers most relevant to users. Payment is not involed as it is with paid search ads.