As usual, Apple has not provided a comprehensive list of fixes, but those it does acknowledge:
- Resolve an issue that may cause Migration Assistant to unexpectedly quit
- Improve compatibility when connecting to a Microsoft Exchange server in Mail
- Address an issue playing audio through a Thunderbolt display
- Resolve an issue that could prevent iMessages from being sent
- Address an issue that could cause the system to become unresponsive when using Pinyin input
- Resolve an issue when connecting to SMB servers with long names
- Address a issue that may prevent Safari from launching when using a Proxy Automatic Configuration (PAC) file
- Improve 802.1X authentication with Active Directory credentials
At the time of writing Apple had not disclosed any security fixes included in the update.
For individual systems, the simplest way to install the update is via Software Update, which (under Mountain Lion) takes you to the Updates tab of the Mac App Store.
Those responsible for multiple computers will usually prefer to download the standalone updater from Apple's support site.