What are those html valid links that I see on site? What's the difference between valid and non-valid html, and what difference does it make for my site?
Html, or hypertext markup language, is a coding language that was developed in the 1980s, and is still the main language that is used on the internet.
It's worth thinking in terms of language. Http is a language that has a well defined grammar - defined by the W3c, or World Wide Web Consortium.
Sites that display those 'valid html' links, like BoduWeb.com (look in the bottom corner of this page), have submitted their code to an online validator, which checks that the code follows the grammar defined by W3c.
Does that make them better web sites?
Well, yes and no! An awareness of the language is essential to any web designer, and so seeing valid html is always a good sign. On the other hand, though, there are certain web effects that, if used, will not produce valid html. A web designer has to balance a client's needs alongside html validity.
One of the most famous examples is Google, and its home page, which fails a html validation test.
Wherever it's possible to achieve the same effects using valid or non-valid html, it's always advisable to use te valid html option - as this will help maintain the design of the page in the long term. For example, new versions of browsers are more likely to render valid html correctly.