This feature doesn’t need much explaining but I did want to address it as it is an important part of a system’s overall security. Keeping your system and applications up to date ensures all the latest security patches are installed as soon as they are available. Of course you can click ‘Software Update’ in the Apple Menu yourself but why not let OS X check for you? [Updated on December 22nd, 2013 to accommodate Mavericks users]
[OS X 10.8 Mountain Lion and earlier] Open your System Preferences and click Software Update. Only a few options are available and it is best to leave them all checked. The 4th option is not important for security purposes but can be convenient.
With these settings enabled OS X will notify you whenever an update is available and ready to install. This ensures you always have the latest patches available even if you forget to check for updates manually through the Apple Menu or the App Store. If your system has been offline for a few days or longer it is still best to check for updates manually rather than wait for the notification to pop up.
[OS X 10.9 Mavericks] For Mavericks users the options have moved to System preferences > App Store. The options are pretty much the same, only “Install app updates” was added. Enabling all options is recommended from a security point of view but you may want to leave app updates off so you can read about changes and possible changes before updating.