If I install ubuntu, can I install the server afterwards? Like, I install ubuntu on one partition then use apt-get or something similar to get the server?
What's the difference between desktop and server?
1) The first difference is in the CD contents. The "Server" CD avoids including what Ubuntu considers desktop packages (packages like X, Gnome or KDE), but does include server related packages (Apache2, Bind9 and so on).
2) The Ubuntu Server Edition installation process is slightly different from the Desktop Edition. Since by default Ubuntu Server doesn't have a GUI, the process is menu driven, very similar to the Alternate CD installation process.
3) Ubuntu server installs a server-optimized kernel by default.
4) Ubuntu Desktop will receive 3 years of support, while Ubuntu Server will be supported for 5 years.
If I install ubuntu, can I install the server afterwards?
Yes, you can do this by donwloading and burning the server cd and installing it in another partition.
Otherwise, you can just use apt-get to install the packages you need like Apache and Bind9 for example.