Hello,
Nowadays, when consumers buy computers, an OS is already installed on the system by the manufacturer at the factory, meaning when you first turn it on you can use it straight away without having to install an OS.
These systems have an OEM version of the OS installed, using a 'Windows Vista OEM Preinstallation Disc' which has been customized by the computer manufacturer to include branding and necessary programs and drivers. So, preinstallation discs are intended for system builders.
A retail copy of an OS in most instances, allows the user to perform a clean install or an upgrade of an OS. It is more expensively priced, because retail versions are coupled with a retail license key, whereas a preinstallation disc doesn't come with a license key.