There are many semantic tags available, and proper use ensure that websites will be well formatted, aesthetically pleasing, user-friendly, and accessible to the differently abled.
Aside from headers, there are a number of other semantic markup techniques available from html. For example:
The image below demonstrates a basic semantic layout in HTML5.
Accessibility covers all of these and more...
Access to Information
Helping Close the Social Divide
The most important semantic tags are grouped as structure tags, textual meaning tags, media type tags, and correlation tags.
Section 508 Accessibility
Section 508, which was codified in 1998, requires that all electronic and information technology developed or purchased by Federal Agencies be accessible by people with disabilities. Functional and technical standards were developed to govern any IS that fall under section 508, and require that a person who has mobility issues, is blind or deaf, or otherwise disabled will be able to effectively use the Information System in question.
SEO (Search Engine Optimization
SEO is the incorporation of keywords and meta tags to bring organic (non-paid) search traffic to your website to increase the quantity and quality of visitors.
SEO and Section 508 are related because they allow users to be presented with quality information in a coherent and understandable way, which provides a better experience for regular users and eases some of the difficulty of surfing the web for people with disabilities since it is much easier to find quality results.