Desmond Tutu has not made any contribution directly to the Ubuntu Linux distribution. He is the author of a book detailing the philosophy of Ubuntu, a South African term meaning humanity towards others. The Ubuntu Linux distribution aims to further such a philosophy using Linux.

See this article: http://en.wikipedia.org/wiki/Ubuntu_(philosophy)

"Ubuntu wiki is a philosophy focusing on peoples allegiances and relationships with each other. This describes a person who is open to others, and has a proper self-assurance of self."

Ubuntu means "an ethic or humanist philosophy focusing on people's allegiances and relations with each other". And the logo is a graphical representation of 3 people holding their arms out, making a circle.

Xubuntu and Ubuntu are two different versions of Ubuntu. (Ubuntu has GNOME.)