What is validation?

One of the W3C's core roles is to set standards and makes code recommendations for the overall web community.
Developers and browser makers should adhere to these standards and recommendations, but they do not.
In the past, browser manufacturers have made up their own tags (e.g. <marquee>) - throwing WC3's recommendations into disarray.
Web developers have often got away with sloppy code practices due to built in tolerance in browsers (e.g. not quoting attributes, not closing table cells etc.)
Web developers have also had to resort to deliberately writing invalid code as "hacks" to get around known browser problems.
We currently have a situation where most sites built today are invalid in the purest sense, meaning that they use code that is not recommended by W3C.