Semantic markup refers to html code and how the code is used to display the information of webpages or web applications. HTML uses unique identifiers which allows them to make the information they are providing easier to read and more user friendly. By looking at the html code the user can see how the page will look like. The semantic markup helps keep the design and visuals of a page seperate from the plain text that will be used on the page.
SEO Search Engine Optimization
Search Engine optimization is how visible to search engine sites such as Google or Bing. These search engine sites use a complex algorithim that collects data such as usability, links, and accessibility of websites. Websites that meet the standards of usablity and that have a lot of links to other sites get ranked higher by Google which makes them more visible on the web. It is very important to have a good ranking SEO!
Section 508 Accessibility
Section 508 Accessiblity are the federal standards that focuses on allowing people with disabilities to have access to the information and technology as persons without disabilities. When developing a website it is important for the developer to have proper semantic markup that considers people with disabilities.