The Terminal (also called shell, bash, console, konsole, xterm, command line, and more) is a gateway into the the inner workings of Linux. It cuts through the GUI
and gets into the "nuts and bolts". Killing frozen applications, starting new ones, copying files, and really anything you can do in Linux can be done through bash commands. If you have ever used MS-DOS, you have an idea about what the terminal does.
To open a terminal, go to the KMenu -> System -> Terminal -> (pick one). The "Super User" terminal lets you operate as root. This is also possible from the others with a "su" command.
SS64.com has a great list of bash commands with explanations and other tidbits, there's another great list here.
Using the terminal is the experienced users choice for operating Linux. It can be faster, easier, and provide more control to an experienced user. New Linux users can still use bash commands, but ease into it. It will take time and patience to learn all the commands and become comfortable with it.
Here is a great introduction to the shell, called Learning the Shell.