This is the 14th release since the project was begun in October 2004 and marks the divergence of the distribution from all the others in many ways.
For one, this is the first distribution to use the Unity interface, rather than sticking with GNOME as it has done from its inception. The GNOME project released its new version, 3.0, recently.
Natty Narwhal also uses Wayland instead of the X server.
The new interface is not automatically installed for all users; on older PCs, if Unity is not supported by the graphics cards, then an older user interface will be presented to the user.
Version 11.04 also includes Banshee, a new application to manage music.
Another feature in Natty Narwhal is that the version can be tested within a browser, without having to install anything.
Ubuntu 11.04 can be downloaded here.