Question

What is W3C Validation?

Answer 1

The W3C Validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc.

Answer 2

W3C is an acronym for World Wide Web Consortium which is often used to verify HTML and XHTML document. This service is very important for browser compatibility, sites usability as well as technical quality of pages.
To get more idea at http://en.wikipedia.org/wiki/W3C_Markup_Validation_Service