From Mac Guides

Software Update is Apple's system for providing Mac users with the latest versions of their Apple software.

Settings

By default, Software Update is set to run automatically once a week. If it finds any updates, it will inform you. If no updates are found, then you will never even notice the program.

To change how Software Update operates, open up the System Preferences and go to "Software Update." Here, you can adjust how often it will check for updates. You can also see exactly which updates have been installed on your system in the past.

Updating Manually

If you do not wish to wait for Software Update to run its check automatically, you can check for updates manually. You can do this by:

• Going to the Apple menu and choosing "Software Update..."
• Clicking the "Check Now" button in the Software Update preference pane in System Preferences
• Going to "About This Mac" in the Apple menu and choosing "Software Update..." in the window that appears

Using Software Update

When there are updates available, the software update window will look something like this:

The little triangle in a circle next to the update indicates that you will have to restart your Mac after installling this update. You can also see how large the update will be, and what version of the software you are downloading.

Installing Updates

To install the updates, simply click the "Install" button at the bottom of the window. All updates that have a "tick" next to them in the "Install" column will be installed. If you wish to install a particular update later, you can simply "untick" it and then install it next time you run Software Update.

After installing any major updates (such as a Mac OS X update), it is generally a good idea to repair permissions.

Ignoring Updates

If you wish to never install a particular update, you can tell Software Update to ignore it. To do this, select the update, and choose "Update : Ignore Update" from the menu bar.

If you later change your mind, you can go to "Software Update : Reset Ignored Updates" from the menu bar to make the update available for download again.