How does SharePoint Foundation 2010 map to WSS 3.0? Is it a replacement for WSS 3.0?
SharePoint Foundation 2010 is the new name for WSS 4.0, it is still free and it gives you the basic collaboration functionalities as WSS 3.0 does.
One of the key changes is the introduction of Business Connectivity Services to SF2010. This feature (previously BDC) allows you to connect to external content sources e.g. databases, web services and present data in SharePoint lists and columns.
Other features are more or less similar to WSS 3.0 just upgraded and polished :)