A "build" is the actual server binary itself. In order to update the build (or upgrade) you have to download and run the installer appropriate for your platform.
After running the installer you should always perform a "Check for Updates". The code that "Check for Updates" downloads (base system, plugins, extensions, etc) can change on a daily basis depending on new features added, bug fixes etc. The installers are not built daily so they can sometimes be behind in their included plugins code.
Hosted customer should periodically "Check for Updates" to make sure they are running the most current plugins and so on. Our Engineering Department takes care of the server itself, but keeping current on plugins is the responsibility of the administrator/owner of the hosted site.