Apple Security Update for Mac
Mac OS X comes with various functions designed to provide a secure working environment: Gatekeeper provides extensive control on what applications are downloaded and installed on your Mac, you can use FileVault to encrypt your disk content, you can decide if you want to share your current location or not, and much more.
Easy to install software package designed to provide the latest OS X security fixes
However, any system service or application can have vulnerabilities that can be exploited by malicious applications. Once this issues are discovered by Apple, a full investigation is conducted and a patch fixing the problems is made available.
Apple provides system software updates that are available free of charge, and are designed to keep your Mac on top of recently discovered security threats. Note that any of the security issues discovered are not made public until there is a fix is provided.
Apple vetted software package that includes the latest security patches
The most recent security fixes made available for your Mac are usually included in the OS X system update, but you can also download the Apple Security Update separately. This way, you can have the latest security updates on your system without installing the OS X upgrade on your computer.
Keep in mind that once a patch for a certain issue is available, Apple also provides a detailed description on what has changed and why. As a result, it is highly recommended to install the latest Apple Security Update as soon as possible.