Windows Update is a utility to keep your operating system up to date. It provides security patches and bug fixes.
Windows Updates can be configured for automatic installation so that updates are always installed. Another option is to have the update download, but wait until you specify when to install the update. Another option is to have the system check for updates but don’t download the update. This option would be useful if you have limited bandwdith. Finally, there is an option to never check for updates.