Sharepoint: Moss vs WSS

In Inside Microsoft Windows Sharepoint Services 3.0 - Ted Pattison & Daniel Larson

"While WSS and MOSS re both pieces of software built by the Microsoft Office team, WSS is included as part of Windows Server 2003 SKU operating system while MOSS is a separate product with its own SKU.You should think of WSS as the underlying platform and think of MOSS as a value-added set of components and services that has been built on top of this platform."