SharePoint Services is a royalty free add-on to Windows Server 2003, and provides business users with multiple web site provisioning, collaboration tools, and document management functionality. Moreover, WSS is a rich development platform providing much added functionality on top of ASP.NET. Version 3.0 is currently in Beta 2 and set to replace the current version 2.0 of WSS in the later part of 2006.
Want to know more about the new Windows SharePoint Services version, then Rob Garrett has writen some nice articles about Windows SharePoint Services 3.0: