Web standards are the technical specifications that dictate how HTML, XML, CSS, XHTML and other technologies used to build web pages should be used and how they should be interpreted.
The web standards are the underlying rules of the internet. Building your web site in line with these standards will help address accessibility and browser compatibility, and will help insure that your web site will continue to work correctly for new versions of web browsers.
Insuring that your web site complies with the web standards also means not having to re-code your web site for new ways to access the web, such as netbooks and smart phones, or whatever comes out next.