Stylesheets Tutorial - WebMonkey
This is a step-by-step tutorial that will introduce you to the basics of Cascading Stylesheets as well as walk you through some of the more advanced features.
Cascading Style Sheets Home Page
The CSS specifications are created by the World Wide Web Consortium, a.k.a. the W3C. These are the same folks who develop the specs for HTML and XML, etc.. This is the official home page for CSS at the W3C website. You can find the official documentation for CSS here as well as an extensive list of links to CSS resources.