Different type of Server 2008 WDS images explained
Boot Image
WDS Boot Image contain Windows Pre-installation Environment (Windows PE). This slim down operating system is used to connect to the WDS Server. The boot image can be found in Server 2008 installation media (search for boot.wim).
Discover Image
WDS Discover Image is created from boot image. This image allows an alternative way to load Windows PE and connect to a WDS server without using PXE boot. It can be stored on a physical media to load Windows PE and connect to WDS.
Capture Image
WDS Capture Image is created from boot image also. This image is used to capture a base system image that is ready for imaging.
Installation Image
WDS Installation Image consists of a file that contains Windows installation files. It make sense to have an Installation Image for every version of OS ( XP, 2003, 2008, etc.)