Many Ubuntu users struggle with the difference between the Ubuntu Desktop and the Server. They really are different animals and that distinction is important if you want to deploy servers in your workplace. I have seen organizations that have installed the Desktop, because they wanted a GUI, and used it as a server. This is because they did not understand the distinction.
When you build servers, they are typically working in a hostile and ever changing environment. This means that servers need to run as few programs as possible to reduce needed resources and enhance security by limiting security issues with unnecessary programs, like the XWindow. Therefore, the major difference between the Desktop Ubuntu and the server is the elimination of the XWindow and all the desktop applications like the word processor and web browser. In addition, servers often use multiple CPUs and larger amounts of RAM which require different kernels. However, you do have the option to install the Desktop and then add all of the server applications so that you can run both. If you want the graphical interface for the server you must install the Desktop and then update to a server kernel.
Security is another distinction. You may or may not have a firewall on your Desktop, you really should, but the server must have a firewall and must employ other security features to protect it from the sure to come attacks that will be placed on it. Passwords, file permissions and user access are all issues of security that must be scrutinized when you have a server.
The server needs more capacity as for networking. As a server will be providing multiple connections at one time to users who are accessing the resources of the server you will need to have faster network cards, good switches and a bigger pipe for network traffic.
The sever needs constant care and evaluation. Servers because of the nature of their being will be accessed by a lot of people so an administrator will need to constantly evaluate the state of the server. This also means that logs will grwo dramatically in /var/log so space will need to be available for that.