Longhorn Server is now-a-days most talked about. The reason behind this is not about the innovative features installed in to it but because of the stripped down version of the server.
It's called Server Core, and it will take only one-sixth of the disk space of a normal Longhorn installation. It is said to be a version of Windows that does not, in fact, use Windows. It is based on the past experience that fewer "moving parts" in an operating system equates to fewer vulnerabilities, stability and security issues. Reducing the amount of code can help reduce the amount of bugs. And that what Server Core is all about.
Server Core can only act as a file server, domain controller, DNS server or DHCP server. Besides these four core roles, Server Core also supports Cluster Server, Network Load Balancing, the Unix subsystem, the new Windows Backup in Longhorn, Multipath I/O, Removable Storage Management, BitLocker drive encryption and SNMP. Server Core also supports Remote Desktop administration, although you'll only get a command-line window when you connect.
Aprt from this, what the administrator would be missing would be an IE, OutLook Express, Calculator or other basic tools and application(of less importance). However, there would also be no Microsoft Framework, which means no managed code on Server Core.
Server Core would include better Remote, non-GUI management tools, requiring smaller disk and memory requirements. It is said that it would be more secure and stable.
Thanks: Don Jones
For more information you can read this article at RedmondMag.com