WCAG's purpose is “Proving a single shared standard for web content accessibility that meets the needs of individuals, organizations, and governments internationally” - WAI WCAG

Background

The internet and the web are world-wide resources. They are not constrained by borders and countries. Because these are world-wide resources, it was important to create shared standards across the world. In 1994 Tim Berners-Lee founded the World Wide Web Consortium (W3C) at the Massachusetts Institute of Technology. The World Wide Web Consortium (W3C) is an international community where member organizations, a full-time staff, and the public work together to develop Web standards which can be used by individuals, organizations, and governments internationally.

"Worldwide, there are more than 750 million people with disabilities. As we move towards a highly connected world, it is critical that the Web be usable by anyone, regardless of individual capabilities and disabilities. The W3C is committed to removing accessibility barriers for all people with disabilities - including the deaf, blind, physically challenged, and cognitive or visually impaired. We plan to work aggressively with government, industry, and community leaders to establish and attain Web accessibility goals." - Tim Berners-Lee

WC3 and WAI

The World Wide Web Consortium’s (W3C) commitment is "to lead the Web to its full potential".

"The social value of the Web is that it enables human communication, commerce, and opportunities to share knowledge. One of W3C's primary goals is to make these benefits available to all people, whatever their hardware, software, network infrastructure, native language, culture, geographical location, or physical or mental ability. To that end they promote ensuring the web has a high degree of usability for people with disabilities." -- W3C Mission

In April 1997 the WC3 created the The Web Accessibility Initiative (WAI) whose mission is to create strategies, standards, and supporting resources to make the Web accessible to people with disabilities.

One of many excellent online resources is the WebAIM website (Web Accessibility in Mind) at "http://webaim.org" . Based at Utah State University, it provides information, training, and practical tools for creating accessible content.

WCAG

One of the most important documents to come from W3C are the Web Content Accessibility Guidelines (WCAG) . These guidelines were created to help define how to make web content more accessible to people with disabilities, with the goal of providing a single shared standard.
Since it's release, WCAG 2.0 has become the internationally recognized benchmark for web accessibility.
The Revised 508 Standards are based on WCAG 2.0.
WCAG 2.0 is based on four principles: Websites should be (1) perceivable, (2) operable, (3) understandable, and (4) robust.

WCAG 1.0

Focused heavily on the techniques for accomplishing accessibility, especially as related to HTML.

WCAG 2.0 and 2.1

Subsequent versions of WCAG (WCAG 2.0 and 2.1) took a different approach: they focused more heavily on the principles of accessibility. By focusing more on principles rather than techniques, these future versions of the guidelines are more flexible, and encourage developers to think through the process of accessibility conceptually.
WCAG 2.0 is based on four principles: Websites should be (1) perceivable, (2) operable, (3) understandable, and (4) robust.

Remember:

When WCAG guidelines are followed they improve usability for everyone.
The Web Content Accessibility Guidelines (WCAG) are the most widely-accepted set of recommendations, and were developed over several years of collaborative involvement by a panel of experts and interested individuals.
The WCAG guidelines are stable, reference-able technical standards.
When followed, the guidelines explain how to make web content more accessible to people with disabilities.

The guidelines are an excellent foundation upon which to build accessible web content, but unless the developers understand the reasons behind the guidelines, they might apply the guidelines incorrectly or ineffectively.

Attribution: [W3C Web Accessibility Initiative (WAI)] (2017, Dec. 4) Introduction to Web Accessibility and W3C Standards

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