Labeled OS X "10.0.1 (Build 4L13)" in the About This Mac information box, the update promises increased performance, stability, and support for third party hardware. According to Apple:
Use Software Update in your System Preferences to get the Mac OS X v10.0.1 software update.
Improvements include better support for third party USB devices, Classic compatibility and overall application stability as well as support for the popular Secure Shell service. For Japanese users, an update to the Epson printer drivers is also available.
Users must run Software Update and first download a new version of the actual Software Update utility, then run Software Update again to download and install the OS X update. You can find more information on Mac OS X at the Apple Web site.