All of the most commonly used Operating Systems in use require constant upgrades and patches from their developers who may have found some weaknesses and vulnerabilities as they go live online with the millions of users everyday. These patches may be allowed to do their thing automatically which means that the Operating system is configured to check for updates and patches when it detects a connection to the internet during a user-specified schedule. Some come in bundles forms such as the Vista SP1 and the previous XP SP1, SP2 and the upcoming SP3. Apple has also released a service pack for their OS X Leopard and the many open-sourced OS systems like Linux constantly have upgrades for specific modules that may need attention.
Most of these upgrades are security and privacy related which has become the number one priority with the growing number and increasing complexity of hacker attacks. Some of these updates are quite publicized but if one wishes to optimize the usage of space on their computers and they know the workarounds of their operating systems, there are quite a lot of patches and upgrades that can be quite useless therefore the user can opt not to install them. Advanced users can opt to have in Windows for example, to have the OS check for updates but let you choose which ones to install and which ones to ignore. This action is advised only for advanced users and if one does not know which system files are vital and may need updating, just leave auto-update do the whole process for you.