You see, while Unity is based on GNOME 2.x, it's not really GNOME, and it's certainly not the latest version, GNOME 3.0. Unity is a shell for GNOME.
Under Unity's hood, there are several technical differences. Instead of GNOME's Mutter windows manager, Unity uses Compiz for the windows manager. On top of this, Ubuntu developers use Zeitgeist, a framework that tracks and correlates relationships between the user's activities to supply applications with contextually relevant data.
The Unity display is meant to make the best use of screen real estate, while still giving you useful information. For example, when you're not using the left-hand Mac OS X like task dock, it shrinks into the left screen edge. The design philosophy behind this is in Project Ayatana. According to Mark Shuttleworth, Ubuntu's founder, there are two main aspects to this: Notifications, the sole purpose of which is to notify you of transient events, and Indicator Menus. These combine persistent awareness of a state with a set of options for modifying that state.
With Unity's indicators, the system's icons comes with controls that enable you to see what's what with your active programs and enable you to work with them. This way you can use an application's functionality without needing to minimize one program and maximize the other. So, for example, "if you're playing music using Ubuntu's media-player Banshee you can use the volume control indicator to select tracks to play rather than going to Banshee. The communications indicator gives you access to all your instant messages and e-mail in the same way, and so on.
As always you can download Ubuntu 11.04 to your PC. By June, if you have a high bandwidth Internet connection, you'll also be able to give the Ubuntu 11.04 desktop a try from the cloud. (Note: you can give Ubuntu 11.04 server a try from the cloud today.)