Ubuntu 11.04 (aka Natty Narwhal) is the 14th release of the Ubuntu Linux distribution. It was released in April, 2011, and will be supported by Canonical until October 2012. Canonical is the company which supports the development of Ubuntu and related software products.
The previous release of Ubuntu was 10.10, (Maverick Meerkat). To see how Natty is different from Maverick, view the Release Notes.