Now, on a graphical desktop this is no problem, as you can just open a new terminal, or a new tab in the same terminal. However, this is less practical on a command-line only system, or one you've remoted into via SSH. But there is a solution, and it's called Screen.
Screen is a terminal multiplexer - it allows you to access multiple terminal sessions inside a single terminal window or remote terminal session (including SSH). Think of it as being like having a tabbed terminal, but in the command line. You can leave Screen running and detach from a session, then reconnect to it later, so for instance if you had an Ubuntu Server install you could connect to it via SSH, issue a command to update the system, then detach and attach again later once it had finished.
Screen is almost certainly preinstalled on your system, but if not you can install it via apt-get in the usual way (sudo apt-get install screen). Once you have it installed, you can start it by issuing the following command:
screenYou're now running screen! You should be confronted by a standard copyright notice (you can remove this by editing /etc/screenrc and removing the hash at the start of the line that reads #startup_message off).
All of screen's internal commands require you hit Ctrl-a, followed by the appropriate key for what you want to do. Try hitting Ctrl-a, then c, and you'll create a new virtual console. Then hit Ctrl-a, then n to cycle to the next virtual console, or Ctrl-a then p to go to the previous one. Ctrl-a, then " will get you a list of all currently running virtual consoles that you can select from using the cursor keys, while Ctrl-a then d will detach you from your current screen session (it will continue to run in the background, so you could potentially log out and leave the processes to run while you do something else). When you want to reattach, just start screen with screen -r and it will reattach you to the running processes. To stop screen, just exit each of the running consoles with Ctrl-d or the exit command in the usual way.
Screen is of limited use on a graphical desktop, but you can still use it there to get used to the idea. It's most useful when dealing with command-line systems. For more details, check out the man page for Screen, or try searching on Google. It's a great tool that makes a lot of tasks easier, and is indispensable if you have to log into a server via SSH a lot. It's also quite handy if you use a lot of command-line tools because it means you can have everything in one terminal session - for instance, you could have mutt in one screen, irssi in another, lynx in a third, Vim in a fourth, and be compiling code using GCC in a fifth, all at the same time, and in a way that makes it easy to switch between them.
It's also worth checking out byobu, which is an Ubuntu-specific tool that makes Screen a lot easier to use if you're new to it. Byobu makes it easy to set Screen up according to your preferences, without having to edit the configuration files by hand.