You should receive an automatic notification asking you to install the update. You can also manually initiate the update process by clicking on the Apple logo on your desktop and selecting 'Software Update'.
The update includes the following fixes and features:
- Safari is updated to 5.1.3
- Adds new language support.
- Addresses issues when using smart cards to log into OS X.
- Resolves issues when authenticating with directory services.
- Resolves a Wi-Fi connection issue when waking from sleep
- Addresses compatibility issues with Windows file sharing.
You can detailed information on this update by visiting - http://support.apple.com/kb/HT5048