The newly released Ubuntu Linux 11.04 has a new default Unity Desktop interface, which brings different experience from the classic GNOME Desktop in all its previous releases which you are already familiar with.
If you dislike this new Unity Desktop for whatever reason, here is the easy way for you to revert back to the Ubuntu classic desktop.
When you are at the Ubuntu login screen, click on the user to login, and you will be able to see the taskbar at the bottom of the screen as shown below. One of the item on the taskbar is the session selector.
Change the session from the default "Ubuntu" to "Ubuntu Classic" and proceed with the login, your desktop will now changed from the new Unity Desktop to the traditional Ubuntu GNOME Desktop. You just need to apply this setting once, and it will be the default for all subsequence login.
If your Ubuntu is set to automatically login the default user, you can get to the login screen by logout the user.