That's easy - Windows SharePoint Services is a free download for Windows Server 2003, Office Sharepoint Server 2007 is a product we sell.
Following my post detailing how to fix the "failed to create configuration database" post about Windows SharePoint Services 3.0 I think it's worthwhile dealing with the often asked question of "What's the difference between Windows Sharepoint Services and Office Sharepoint Server 2007".
The best place to find out which version of Sharepoint you'll need is the excellent Which SharePoint technology is right for you? which runs through every feature and every variant of the product in a handy tabular format. In a nutshell Windows Sharepoint Services 3.0 is an optional download for Windows Server 2003 which provides the basis for web based collaboration. As I explained in my previous post there are practical scenarios which can be implemented purely using Windows Sharepoint Services.
As described in the Which SharePoint technology is right for you? guide there's are a wide range of variants of Sharepoint which bring additional features including enterprise search and business intelligence.
The currently shipping versions of Sharepoint are named as follows:
Windows SharePoint Services 3.0, SharePoint Server 2007 for Search, Forms Server 2007, Sharepoint Server 2007 Standard CAL, SharePoint Server 2007 Enterprise CAL or for Internet sites