Basically, there are two different ways to start a shell from the graphical user interface which usually shows after you have booted your computer:
you can leave the graphical user interface or
you can start a terminal window within the graphical user interface.
While the first option is always available, you can only make use of the second option when you are logged in to a desktop such as KDE or GNOME already. Whichever way you choose: there is always a way back and you can switch back and forth between the shell and the graphical user interface.
If you want to give it a try, press Ctrl+Alt+F2 to leave the graphical user interface. The graphical user interface disappears and you are taken to a shell which prompts you to log in. Type your username and press Enter. Then type your password and press Enter. The prompt now changes and shows some useful information as in the following example:
The hostname of your computer.
Path to the current directory. Directly after login, the current directory usually is your home directory, indicated by the tilde symbol, ~.
When you are logged in at a remote computer the information provided by the prompt always shows you which system you are currently working on.
When the cursor is located behind this prompt, you can pass on commands directly to your computer system. For example, you can now enter ls -l to list the contents of the current directory in a detailed format. If this is enough for your first encounter with the shell and you want to go back to the graphical user interface you should log out from your shell session first. To do so, type exit and press Enter. Then press Alt+F7 to switch back to the graphical user interface. You will find your desktop and the applications running on it unchanged.
When you are already logged in to the GNOME or the KDE desktop and want to start a terminal window within the desktop, press Alt+F2 and enter konsole (for KDE) or gnome-terminal (for GNOME). This opens a terminal window on your desktop. As you are already logged in to your desktop the prompt shows information about your system as described above. You can now enter commands and execute tasks just like in any shell which runs parallel to your desktop. To switch to another application on the desktop just click on the corresponding application window or select it from the taskbar of your panel. To close the terminal window press Alt+F4.