CSS 2.1 makes corrections to the older CSS 2.0 which was standardized back in 1998. This update removes some features that went unimplemented in major browsers, changes those needing corrections and adds a few features as well. Some CSS 2.0 features that become obsolete with CSS3 have also been removed. For most people this does not mean much, since CSS 2.1 is already supported by most browsers, and just makes it official.
With CSS 2.1 out the door, the focus will now shift to the numerous CSS3 modules that are in development, and have been the focus of a lot of buzz recently. The way CSS3 is being developed is quite different from earlier versions. Instead of one major standard with numerous changes, the standard has been modularized, so development on different modules can happen in parallel, and they can be standardized in parallel.
In fact, CSS 2.1 is not the only only new recommendation, the CSS3 Color module has also become a W3C recommendation. It is the first module amongst CSS3's many modules to achieve this status. Another recommendation is a MathML for CSS profile.
You can find out more from the official press release by the W3C.