CSS3 is the latest and there are some very cool features like rounded box corners, text shadows, opacity etc. Comes with a warning about cross-browser compatibilities tho. A good resource for describing the new features and testing various browsers is [url]www.css3.info[/url]
Also, the W3 validator now validates CSS3 pages too:
It depends on what you mean by "latest": The latest specification is css2. The latest recommendation is 2.1, which revised css2, and (without going into the historical detail) is commonly treated as if it was the "latest".
css3 is the next version, and has broken css into "modules". Most are still in draft status although modern browsers have already started to implement many of the features. If you want to follow progress, css3info [css3.info] keeps a handy list.
css is not a validator. Html provides the stucture for a document (eg a paragraph is marked up using <p> ), and css allows you to style those elements. It can be used to style, eg, XHTML and xml as well as html.
You can read most of what I have said in the abstract of the css2.1 [w3.org] recommendation which has lots more information, but css concepts [reference.sitepoint.com] is easier to read ;)