As I get asked the question all the time I think the best answer I can give is to have look at “Which SharePoint technology is right for you” from Microsoft that provides a side by side comparison of all the different flavours of SharePoint. Be warned there is a lot of information to digest here. Perhaps an easier way to understand the different features set is to download the SharePoint product comparison spreadsheet that sets out the comparison using Excel which makes it much easier to understand.
In all of this don’t forget that a new version of SharePoint is on the way which will mean changes again to these charts (hopefully not too much).
One thing that all these documents don’t cover is the price. In simple terms, Windows SharePoint Services is available as a free download, while Microsoft Office SharePoint Server requires purchasing the appropriate licensing.
Hope that makes things a little easier to understand.