The W3C or World Wide Consortium is an international consortium dedicated to creating web standards and guidelines: "To lead the World Wide Web to its full potential by developing protocols and guidelines that ensure long-term growth for the Web." As a group they work towards setting standards and goals for all websites to meet that encourage accessibility on the web. Similar to the way companies can become ISO 9001 certified, websites that meet W3C criteria conform to standards for web quality and accessibility.
Web accessibility follows the principles and ideals of making web pages navigable and visible to all people regardless of physical handicaps, browser, platform, or operating system. The best part of this is that more people being able to navigate and view the pages means more people hitting the website and getting more information out of it, which is what setting up a website is all about!