One of the things that separates Windows from Linux or Ubuntu, a Linux distribution, is the way the operating system handles the graphical user interface. Microsoft Windows is an integral part of the operating system. It is integrated with the operating system and at this point, Windows 7, you can't operate the computer with out the Window's graphical interface. To be fair, you can still get to the old DOS command line by typing cmd in the run window, but it's more there for a link to nostalgia, than it is to do day to day operations. In a lot of ways, I miss the DOS command line, I was good at it. Alas, even some of the old DOS commands are missing, like fdisk, for example. That's all done through the windowing system, now.
In contrast, Linux has two windowing systems, KDE, and the one used in Ubuntu, Gnome. These two windowing systems are independent applications, and more important, they run as applications, just like OpenOffice or a game would run.
Linux consists of a kernal, and another layer that runs user applications, and never the twain shall meet. The kernal uses its own memory space, and each application runs in its own memory segment. If an application needs to access a disk drive, it calls the kernal, the kernal accesses the drive, and passes the information over to the calling applications memory space. Because of this the desktop can crash, say from a video game that has a bug, and it will not bring the computer down. It will just shut down that application, which you can restart from the Linux command line.
In contrast, Microsoft integrates windows with the operating system. So if you have a problem with your video driver, the entire system crashes, and you have to reboot the computer. This is one of the reasons Linux is more secure than Windows.
I mentioned you can restart an application from the command line in Ubuntu. How do we get there? In Windows, you type cmd in the run window, and you get a black window with the command prompt. Ubuntu is pretty much the same. From the Gnome desktop, in the upper left corner, go to Applications->Accessories->Terminal. You'll find a nice black window pop up with a $ prompt. Welcome to the Ubuntu Linux distribution command line.
Putting an Icon on Your Desktop
As a digression, I am at the command prompt much more often than in Windows, so I put an icon on my desktop to bring it up quickly. To put an icon on your desktop, go back to Applications->Accessories->Terminal, but instead of left clicking to bring up the window, right click, and your given a couple of choices. You can add an icon to the laucher panel, this is a tiny icon that appears on the top bar of your screen, like the current Firefox icon, or you can add the icon to your desktop.
There is another choice, of putting it in the menu, as a drawer, the equivalent of a folder in Windows, or a an actual menu item. Since we're making an icon for the desktop, we don't need to put it in the menu, since it's already there. By the way, if you would like to rearrange your menus, change the drawers, or remove some menu items, the menus are completely configurable. Go to the Applications choice on the top menu bar, for example, right click and select "Edit Menus."
Back to the terminal window, the terminal window gives you complete access to the Ubuntu distro, i.e. the Linux operating system command line, and its plentiful commands, but that is the subject of another post. As a teaser, I'll give you a couple of commands to get you started. Everything is in lower case, type "pwd" for print working directory. It tells you where you are. Type, "ls" to list the files in that directory, or better "ls -al" to get a long alphabetical listing. Some things come from DOS. Type "cd .." for change directory to move up one level in the directory structure, and or to go down, type the name of the directory , for example, "cd home" goes down to the home directory, that should keep you busy for awhile. Have fun and enjoy Ubuntu.
And finally, less we get carried away, you can still explore your file system through the GNOME graphical user interface, like Windows Explorer, without using the terminal window or the command line, by going to the "Places" menu in the upper tool bar.