As HTML5 is to HTML 4, CSS3 is to CSS2. It is the next incarnation of cascading style sheets bringing with it several improvements, such as namespaces, regions, filters, and conditional styles.
And yet again, as with HTML5, the depth and breadth of CSS3 is enough to fill up a series of books. In fact, it is so large that the working group for CSS3 has broken the standard up into a series of modules you can view at http://www.w3.org/Style/CSS/current-work.
We will focus on media types, selectors, and media queries as they are applicable to mobile development, but before we start I would like to remind you all to remember when dealing with CSS the C means cascading. Styles will be applied to elements in the order they appear. Be precise with your styles and make sure your elements and styles have meaningful names.