Terminal is Mac's version of the Command Line: a text-based program that allows you to send commands and receive output.  It doesn't have all the flash of Graphical User Interface (GUI), but it does have the advantage of speed and power.  Basic command line skills are your passport into the world of deep nerd, since many of the real tools used by programmers and developers are command line tools. These links describe the basics of using Terminal, and show you a few fun things on your computer you probably didn't even know about.

Much more detailed information about Terminal: homepage.mac.com/rgriff/files/TerminalBasics.pdf 

Here's some info on the Windows command line: Basic Hints for Windows Command Line Programming 

There's one command that was left out of the "useful commands" that I use all the time: ping.  Try typing ping google.com into the Terminal.  This will tell you if your computer is able to reach that website, and give you an idea of how long it takes to send and receive information.  This is a really quick way of testing out your Internet connection.  Happy pinging!