Video improvements include better single-pass H.264 encoding quality and better playback of Motion JPEG content.
For audio, Apple reports that encoding fidelity has been improved for MPEG 4 AAC (Advanced Audio Codec, the successor to MP3) playback, the default option for ripping in iTunes. Release notes also state that audio tracks from MPEG video files now export consistently.
The new update also includes compatibility improvements for iChat and Photo Booth.
A series of security fixes are also part of the update, which benefits both Mac OS X and Windows users. Many of the security patches address issues with handling maliciously crafted media files, based on security vulnerabilities discovered by third party researchers.
Alternatively, you can download and install the update via Mac OS X's Software Update mechanism accessible via the Apple menu.