in order for your web site to reach the widest audience, it must consider the many individuals who are physically challenged in some way. Color blindness, poor eyesight, deafness and other disabilities should not be a hindrance in visiting your web site.
SEO is the technique of improving traffic via "natural" search engine results for targeted keywords. There are many techniques, both subtle and complex, that can improve your web site ranking and increase your traffic.
usability involves the science of human interface engineering, user behavior, content optimization and other aspects of improving the user experience of your web site. Key to this concept is the simple mantra "don't make me think".
validation is the process of testing an aspect of a web site against the official standards of a governing body. In the case of a web site, these standards are often set by the W3C. XHTML, accessibility, CSS (style sheets) and other standards are an important part of overall site success.
XHTML is one of the most important standards in web site design. Technically, it means html documents are XML-compatible. This makes web sites much more cross-browser, search engine, and standards-friendly, and means that web sites can be processed by software other than web browsers (mobiles, screen readers for the visually impaired, etc.).