Web Part Pages is the name of the rendering technology used in the 2003 version of SharePoint Products and Technologies. web Part Pages are used to host Web Parts, the successor to SharerPoint Portal Server 2001 dashboard web parts. (BTW, Web Part Pages are included as native components in the forthcoming release of ASP.Net 2.0 - i.e. they're strategic to Microsoft.)
A SharPoint Site is a collection of Lists and Web Part Pages. List are used to store data/information and Web Part Pages are used to display lists using a collection of one or more Web Parts.
There is a table in the SharePoint Sites and Site Collections section of Technical Overview white paper that addresses some of the other SharePoint 2003 terminology differences. Checkout www.microsoft.com/.../spST23TO.asp#e2.