Server Core is the small footprint version of Windows Server 2008 - around 1GB footprint on the disk. With that comes a lot smaller attack surface etc. But originally it did not include IIS7.0. Last week we announced at Tech Ed that IIS7.0 would be included which is great news. Alas it will not include ASP.NET or the .NET Framework. Shame - but folks are working on adding that in the future.
Plenty more detail from Bill.