From the introduction on the Ubuntu web site (http://www.ubuntulinux.org/):

"Ubuntu" is an ancient African word, meaning "humanity to others". Ubuntu also means "I am what I am because of who we all are". The Ubuntu Linux distribution brings the spirit of Ubuntu to the software world.

Ubuntu is a complete Linux-based operating system, freely available with both community and professional support.

The Ubuntu community is built on the ideas enshrined in the Ubuntu Manifesto: that software should be available free of charge, that software tools should be usable by people in their local language and despite any disabilities, and that people should have the freedom to customise and alter their software in whatever way they see fit.

These freedoms make Ubuntu fundamentally different from traditional proprietary software: not only are the tools you need available free of charge, you have the right to modify your software until it works the way you want it to.