what is the difference between ubuntu and ubuntu server? aside from server being command line only
are there services/software that exist on server that dont on the desktop edition?
• The Server Edition uses the Deadline I/O scheduler instead of the CFQ scheduler used by the Desktop Edition.
• Preemption is turned off in the Server Edition.
• The timer interrupt is 100 Hz in the Server Edition and 250 Hz in the Desktop Edition
Other than that, you can essentially convert any version to anything else, by installing the right packages/package bundles
Update: there's no longer seperate server and generic kernel types. This answer is no longer technically correct, though at the time of this question and answer, it was.
If you look at any flavor of Linux, you will notice that they all start from a kernel. From there, software is added (typically in packages). Each of the flavors has put together packages of software that they think will serve your purpose. Canonical is responsible for Ubuntu, and they use their community to make decisions on which packages make it into each distribution.
Ubuntu and Ubuntu Server are no different. They have software packaged together to create a distribution. The difference is that the Ubuntu community packaged Ubuntu Desktop to be used primarily as a desktop and Ubuntu Server to be used primarily as a server.
Of course, you can always add the desktop packages to server and vice versa.
I hate to put it so simply, but that is how they intended it to be.
EDIT: All the services/software exist on both desktop and server. You can even put the desktop on Server if you want to.