During installation, you were asked to create a hostname for your server. The default during installation is ubuntu, but you can (and should) come up with your own name. If you left the default, or if you want to practice changing it, we'll work through that in this section.
In most organizations, there is a specific naming scheme in place for servers and networked devices. I've seen quite a few variations, from naming servers after cartoon characters (who wouldn't want a server named daffy-duck?), to Greek Gods or Goddesses. Some companies choose to be a bit boring, and come up with naming schemes consisting of a series of characters separated by hyphens, with codes representing which rack the server is in, as well as its purpose. You can create your own naming convention if you haven't already, and no matter what you come up with, I won't judge you.
Your hostname identifies your server to the rest of the network. While the default ubuntu hostname is fine if you have...