CSS (Cascading Style Sheet) is used to make the site more managable by providing designers a way to design the site using some external stlye sheet classes that take care about the look and feel of the website. In this way the look and feel of the whole web site can be managed very easily and the maintenance is less.
Now its the time of revolution as the new term Web2.0 is evolving and
CSS 2 (an step ahead to CSS) is the part of this new term.

What is CSS 2

CSS2 is a style sheet language that allows authors and users to attach style to structured documents by separating the presentation style of documents from the content of documents, CSS2 simplifies Web authoring and site maintenance.
CSS2 builds on CSS1 that means all valid CSS1 style sheets are valid CSS2 style sheets. CSS2 also supports media-specific style sheets so that document presentation can be tailored to visual browsers, printers, handheld devices, etc. CSS2 specification also supports
-->content positioning
-->downloadable fonts
-->table layout
-->features for internationalization
-->automatic counters and numbering
-->properties related to user interface.

You can have the CSS2 specification from here.